Syria's foreign minister said Tuesday his country would defend itself using ``all means available'' in case of a U.S. strike, denying his government was behind an alleged chemical weapons attack near Damascus and challenging Washington to present proof backing up its accusations.

The United Nations said that its team of chemical weapons experts in Syria delayed a second trip to investigate an alleged poison gas attack near Damascus by one day for security reasons.

Walid al-Moallem, speaking at a press conference in Damascus, likened U.S. allegations that President Bashar Assad's regime was behind a purported poison gas attack to false American charges that Iraq possessed weapons of mass destruction before the 2003 U.S.-led invasion of that country.

``They have a history of lies _ Iraq,'' he said. Al-Moallem spoke a day after U.S. Secretary of State John Kerry said there was ``undeniable'' evidence of a large-scale chemical attack likely launched by Assad's regime.

Kerry's comments and tough language Monday laid out the clearest argument yet for U.S. military action in Syria, which, if President Barack Obama decides to order it, would most likely involve sea-launched cruise missile attacks on Syrian military targets.

Support for some sort of international military response was likely to grow if it is confirmed that Assad's regime was responsible for the Aug. 21 attack that activists say killed hundreds of people. The group Doctors Without Borders put the death toll at 355.

Obama has yet to say how he will respond, but appeared to be moving ahead even as the U.N. team on the ground in Syria collected evidence from the attack.

Meanwhile, British Prime Minister David Cameron recalled Parliament on Tuesday for an urgent discussion on a possible military response.

At the Syrian news conference, Al-Moallem called the U.S. accusations ``categorically false.''

``I challenge those who accuse our forces of using these weapons to come forward with the evidence,'' he said. Syria would fight back in case a U.S. strike, he added.

``We have the means to defend ourselves and we will surprise everyone,'' he told reporters the Damascus news conference. ``We will defend ourselves using all means available. I don't want to say more than that,'' he added.

He also blamed the postponement of the U.N. team's planned visit to the eastern Ghouta suburb on disputes between rebel gunmen who could not agree on safety guarantees for the investigators.

The U.N confirmed the one-day delay, saying only it was for security reasons. A statement said the decision was made Tuesday in order to improve preparedness and safety, after unidentified snipers opened fire on the team's convoy on Monday on a similar trip to the region.

``The Secretary-General again urges all sides in the conflict to give safe passage and access to the team,'' the statement said.

The U.N. team traveled Monday to the western Damascus suburb Moadamiyeh, one of the areas affected by purported chemical attack, where they collected samples and testimony after a treacherous journey through government and rebel-held territory. Their convoy was hit by snipers but members of the team were unharmed.

U.N. Secretary-General Ban Ki-moon said he had instructed U.N. disarmament chief Angela Kane in Damascus ``to register a strong complaint'' with both the Syrian government and opposition representatives for the convoy attack.

In Geneva, U.N. spokeswoman Alessandra Vellucci told reporters that the inspection team might need longer than the planned 14 days to complete its work and its priority now is to determine what chemical weapons might have been used in the Aug. 21 attack. ``This is the first priority,'' she said.

An Associated Press photographer outside the team's hotel in Damascus said earlier that he saw Kane and Swedish chemical weapons expert Ake Sellstrom leave the hotel while other inspectors stayed behind. (AP)