UNITED NATIONS (AP) — The United Nations has asked the head of its chemical weapons inspection team to expedite the analysis of tests from samples collected from Syria last week, the world organization's top spokesman said Sunday.

Time is of the essence because President Barack Obama announced Saturday that he would seek congressional authorization for a military strike on Syria, which it accuses of killing 1,429 of its own people, including more than 400 children, in an Aug. 21 chemical weapons attack in a suburb of Damascus. Congress is scheduled to return from a summer break on Sept. 9.

The U.S. has said it has done its own testing of samples of hair and blood provided to Washington by first responders in Syria, and Secretary of State John Kerry asserted Sunday that the U.S. now has evidence of sarin gas use, making the case for a strike against the Assad government "stronger by the day."

The United Nations maintains that its team, which follows standards recognized by the Organization for the Prohibition of Chemical Weapons, is uniquely capable of testing samples in an impartial manner, and that any attack on Syria should come with broad international support. However, the U.N. Security Council remains deeply divided on the issue, and any vote on a strike against Syria would all but certainly be vetoed by its ally Russia — essentially paralyzing the U.N.'s most powerful body.

"The secretary-general took note of the announcement by President Obama yesterday on the referral to Congress. I can tell you he regards it as one aspect of an effort to achieve a broad based international consensus on measures in response to any use of chemical weapons," U.N. spokesman Martin Nesirky said. "Use of chemical weapons will not be accepted under any circumstances, and there should be no impunity and any perpetrators of such a horrific crime against humanity must be held accountable."

He added: "The U.N. investigation mission should be given an opportunity to succeed."

Nesirky said that Secretary-General Ban Ki-moon spoke with the head of the U.N. chemical weapons inspection team, Ake Sellstrom, earlier Sunday and asked him to accelerate the testing process, "and to report the results to him as soon as possible."

Nesirky also said that two Syrian government officials are observing the process as mandated by the guidelines that safeguard the samples' chain of custody. He declined to say whether they would be accompanying the samples, which the team collected at the site of the purported attack in the Damascus suburb of Ghouta, to the various laboratories.

They will be sent Monday to laboratories around Europe to check them for traces of poison gas.

Nesirky declined to say when the results might be in. "We are not giving a timeline," he said.

On Saturday, Ban met in New York with U.N. disarmament chief Angela Kane who had just returned from Damascus after days of tense negotiations with Syrian officials over the U.N. probe into the apparent poison gas attack.

The inspectors left Syria and flew out of Lebanon to the Netherlands early Saturday after carrying out four days of inspections.

Nesirky has said that under an agreement between Damascus and the United Nations the inspection team's mandate is limited to determining whether chemical weapons were used rather than trying to assign blame for the attack.