JERUSALEM (AP) — Prime Minister Benjamin Netanyahu on Wednesday warned archenemy Iran not to test Israel amid escalating regional tensions as the country marked its Holocaust Remembrance Day in memory of the 6 million Jews systematically killed by Nazi Germany and its collaborators during World War II.

Netanyahu's comments came as the region braces itself following a tweet by President Donald Trump earlier in the day warning Russia to "get ready" for a missile attack on its ally Syria, suggesting imminent retaliation for a suspected Syrian chemical weapons attack that killed dozens of civilians last weekend.

A U.S. strike in Syria could have implications for Israel. Israel is also on high alert after Iran, a strong ally of President Bashar Assad, threatened to respond to an airstrike on a Syrian military base on Monday that the Syrian government, Russia and Iran blamed on Israel.

"Events of recent days teach that standing up to evil and aggression is a mission imposed on every generation," Netanyahu said in a speech at the Yad Vashem Holocaust memorial center before hundreds of Holocaust survivors and their families, Israeli leaders, diplomats and others.

"We saw the Syrian children slaughtered by chemical weapons, our hearts were torn from the horrific images," he said.

Netanyahu said a big lesson of the Holocaust is that "murderous evil" that goes unchallenged "spreads quickly and gradually threatens all of humanity."

"Today as well, a murderous regime threatens us, threatens entire world peace, this regime explicitly declares that it intends to annihilate us, the Jewish state," he said, in a reference to Iran.

Israel considers Iran an existential threat because of its nuclear and missile programs, its support of violent anti-Israel groups in the region and frequent calls for destruction of the Jewish state.

Tehran insists its nuclear program is for peaceful purposes only, such as power generation.

Netanyahu vehemently opposes the 2015 deal that curbed Tehran's uranium enrichment levels — a potential pathway to nuclear arms — in exchange for the lifting of international sanctions. But the Israeli leader has insisted that the agreement won't stop Iran from developing nuclear weapons.

Drawing comparisons to European appeasement of Hitler before World War II, Netanyahu said the nuclear deal "released the Iranian regime from its chains and since has devoured country after country, similar to what happened in Europe in the 1930's."

He said history shows how "agreements with these kind of regimes were not worth the paper they were written on."

Netanyahu has been a leading critic of the deal, saying it did not contain sufficient safeguards to prevent Iran from reaching nuclear-weapons capability or address Iran's other activities across the region or its long-range missile program.

"I have a message to the rulers of Iran," Netanyahu said. "Do not put to test Israel's resolve."

The ceremony at Yad Vashem, held after sunset, ushered in one of the most melancholy days on Israel's calendar.

Places of entertainment and cafes close down. TV and radio stations broadcast documentaries about the Holocaust and interviews with survivors or somber music until sundown the next day.

According to the Hebrew calendar, the Holocaust Remembrance Day falls on the same date as the Warsaw Ghetto uprising — the ultimately doomed revolt that played such an important role in defining Israel's national psyche.

Israel's identity is defined by the axiom that never again will Jews be helpless in the face of annihilation.

On Thursday morning, Israelis will come to a mournful, two-minute standstill to remember the dead as a siren wails across the country. Pedestrians stop in their tracks. Cars pull over on highways and roads and many people exit their vehicles to stand still in contemplation.

The names of Holocaust victims are read out in parliament.

A study released hours before Wednesday's opening ceremony found that violent attacks on Jews worldwide dropped in 2017, despite a rise in other forms of anti-Semitism, in a year characterized by normalization and mainstreaming of anti-Semitism not seen in Europe since World War II.

Researchers at Tel Aviv University said assaults specifically targeting Jews fell 9 percent last year. They recorded 327 cases, compared to 361 in 2016, which had already been the lowest number in a decade.

But they noted attacks were far more brazen. Most dramatic were a pair of cases in France, where a Jewish woman was thrown to her death out of her apartment window and a Holocaust survivor was stabbed and burned to death in her Paris home.

Threats, harassment and insults have also driven thousands of French Jews to relocate.

Tel Aviv University's Kantor Center for the Study of Contemporary European Jewry releases the report every year on the eve of Israel's Holocaust memorial day.

Increased security measures are credited with reducing violence, but it may be masking a trend of anti-Semitism becoming more mainstream and acceptable, particularly in European politics. The report described a toxic triangle made up of the rise of the extreme right, radical Islamism and a heated anti-Zionist discourse on the left accompanied by anti-Semitic expressions.