TEHRAN, Iran (AP) — Iran and Russia on Friday backed a military campaign to retake the last rebel-held stronghold in Syria as Turkey pleaded for a cease-fire, narrowing the chances of a diplomatic solution to avoid what many say would be a bloody humanitarian disaster.

The trilateral summit in Tehran involving Iranian President Hassan Rouhani, Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan puts further pressure on the rebel forces still operating in Syria's northwestern Idlib province, including about 10,000 hard-core jihadists and al-Qaida-linked fighters.

It left the chance, however slim, for further diplomacy to try to separate civilians and rebels from the Islamic militants in Idlib.

While Putin called for the "total annihilation of terrorists in Syria," he left open the possibility of a cease-fire. Rouhani as well spoke of "cleansing the Idlib region of terrorists," while also noting the need of protecting civilians.

Turkey's President Recep Tayyip Erdogan and Russia's President Vladimir Putin greet each other in Tehran, Iran, prior to their Syria talks. (Kirill Kudryavtsev/Pool Photo via AP)

Turkey, which backed opposition forces against Syrian President Bashar Assad, fears a military offensive will touch off a flood of refugees and destabilize areas it now holds in Syria. Ankara also has hundreds of troops manning 12 observation posts in Idlib.

"Idlib isn't just important for Syria's future; it is of importance for our national security and for the future of the region," Erdogan said. "Any attack on Idlib would result in a catastrophe. Any fight against terrorists requires methods based on time and patience. . We don't want Idlib to turn into a bloodbath."

Erdogan also sought to use Persian literature to drive home his point in Tehran, quoting the poet Saadi: "If you've no sympathy for human pain, the name of a human you cannot retain."

The U.S. also warned against an assault in Idlib, with Ambassador Nikki Haley telling the U.N. Security Council that "the consequences will be dire."

Northwestern Idlib province and surrounding areas are home to about 3 million people — nearly half of them civilians displaced from other parts of Syria.

For Russia and Iran, both allies of the Syrian government, retaking Idlib is crucial to complete what they see as a military victory in Syria's civil war after Syrian troops recaptured nearly all other major towns and cities, largely defeating the rebellion against Assad.

A bloody offensive that creates a massive wave of death and displacement, however, runs counter to their narrative that the situation in Syria is normalizing, and could hurt Russia's longer-term efforts to encourage the return of refugees and get Western countries to invest in Syria's postwar reconstruction. Russia also wants to maintain its regional presence to fill the vacuum left by the U.S. and its long uncertainty over what it wants in the conflict.

"We think it's unacceptable when (someone) is trying to shield the terrorists under the pretext of protecting civilians as well as causing damage to Syrian government troops," Putin said. "As far as we can see, this is also the goal of the attempts to stage chemical weapons incidents by Syrian authorities. We have irrefutable evidence that militants are preparing such operations, such provocations."

Putin offered no evidence to back his claim. The U.N. and Western countries have blamed Assad's forces for chemical weapons attacks in the civil war, something denied by Russia and Syria. The U.S., Britain and France have vowed to take action against any further chemical attacks by Assad's regime.

Reacting to Erdogan's proposal for a cease-fire in Idlib, Putin said "a cease-fire would be good" but indicated that Moscow does not think it will hold.

An activist and resident of Idlib, with protesters chanting as they holding Syrian revolution flags, in Harim, a town in Idlib province, Syria. (Courtesy of Mustafa Alabdullah via AP)

"We hope that we will be able to reach an agreement and that our call for reconciliation in the Idlib area will be heard," the Russian president said. "We hope that the representatives of those terrorist organizations will be smart enough to stop the resistance and lay down arms."

There was no immediate reaction from fighters in Idlib. Naji al-Mustafa, a spokesman for the Turkey-backed National Front for Liberation, said before the summit that his forces were prepared for a battle that they expect will lead to a major humanitarian crisis.

"Idlib is about a lot of international power play and everyone is looking after their interests," al-Mustafa said.

Syria's U.N. ambassador, Bashar Ja'afari, told the Security Council that the government is determined to regain Syrian territory and "liberate it from terrorism and foreign occupation." He said countries that facilitated the entry of foreign fighters, especially Turkey, "still have a chance to remove them from Idlib province."

Staffan De Mistura, the U.N. envoy for Syria, told the council that "the signals" from the Tehran meeting are that they intend to continue talking to avoid a potential catastrophe in Idlib. Ideally, he said, all fighters should be given a deadline to move out of populated areas and all air and ground attacks on population centers should be halted, with Russia and Turkey especially but also Iran standing as guarantors of the plan.

De Mistura said a military offensive in Idlib would be incompatible with U.N. efforts to form a committee to draft a new Syrian constitution. It would be a failure of diplomacy "if with these efforts we simply saw an increase of military activities," he said.

Early Friday, a series of airstrikes hit villages in southwest Idlib, targeting insurgent posts and killing five people, including a civilian, said Rami Abdurrahman, the head of the Britain-based Syrian Observatory for Human Rights. Abdurrahman said suspected Russian warplanes carried out the attack.

Faysal al-Antar, a member of the local council in Kfar Zita, one of the towns on the southern edge of Idlib that was hit in the airstrikes, said warplanes were flying as the leaders convened Friday in Tehran.

"The meetings never translate on the ground," he said. "Imagine there is a meeting to calm the situation, while we are being hit, and there are airstrikes as it takes place. If they had the slightest respect, they would have at least halted the strikes for the duration of the meeting."

Already, nearly a half-million people have been killed in the grinding civil war, which began first as a popular uprising against Assad and later devolved into a sectarian and regional conflict.

Eight aid agencies warned that in the coming offensive "it will be the most vulnerable who will pay the heaviest price, with women, children, and the elderly in Idlib unlikely to be able to move to safety."

But Hassan Hassan, a Syria expert and a fellow at the Washington-based Tahrir Institute for Middle East Policy, said he is doubtful an offensive is imminent, pointing to Turkey getting U.S. backing in opposing a major offensive.

"The regime might conduct a face-saving attack on areas away from Turkey's zones of operation, a low-hanging fruit," he said. "I say this because the US is making it clear it is not bluffing this time, and Turkey is similarly against the offensive."

Rallies were a day of protests against Syrian President Bashar Assad and his troops' imminent offensive against Idlib, the last bastion of rebels in Syria. (Courtesy of Wissam Zarqa via AP)

In her remarks at the U.N. Security Council, Haley said the U.S. has been clear with Russia and other nations that "we consider any assault on Idlib to be a dangerous escalation of the conflict in Syria."

"If Assad, Russia and Iran continue, the consequences will be dire," she said.

"We urge Russia to consider its options carefully. Stop Assad's assault on Idlib. Work with us and the U.N. to find peace at last for Syria," she said.

The U.S. has found itself largely on the sidelines of the possible offensive as Iran, Russia and Turkey — all nations that Washington has imposed sanctions upon — discuss Idlib's future. Although the U.S. has about 2,000 troops and outposts in Syria, President Donald Trump has said he wants to pull those forces out after the war against the Islamic State group dislodged the extremists from vast territories it once held there and in Iraq.

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Associated Press writers Amir Vahdat in Tehran, Zeina Karam and Sarah El Deeb in Beirut, Edith M. Lederer at the United Nations, Nataliya Vasilyeva in Moscow and Suzan Fraser in Ankara, Turkey, contributed to this report.