DAMASCUS, Syria — U.N. chemical weapons experts headed to a Damascus suburb today for a new tour of areas struck by a purported poison gas attack, activists said, as the U.S. laid the groundwork for a possible punitive strike and the U.N. chief pleaded for more time for diplomacy.

U.S. leaders, including Vice President Joe Biden, have charged that Syrian President Bashar Assad’s government fired deadly chemical weapons near Damascus last week — though they have not presented concrete proof and U.N. inspectors have not endorsed the allegations.

Syria, which sits on one of the world’s largest stockpiles of chemical weapons, has denied the charges.

Still, world powers appear to be moving toward a strike against the Assad regime, including by readying possibly legal justifications.

Britain said it will put forward a resolution to the U.N. Security Council on Wednesday to condemn the Syrian government for the alleged attack.

Prime Minister David Cameron’s office said Britain would seek a measure “authorizing necessary measures to protect civilians” in Syria under Chapter 7 of the U.N. charter. Military force is one of the options that can be authorized.

Any language that could be read as allowing a military strike is likely to face veto by Damascus allies Russia and China.

In Geneva, U.N.’s special envoy to Syria Lakhdar Brahimi said any military strike must have U.N. Security Council approval.

Britain’s parliament is to hold an emergency session Thursday on a motion clearing the way for a British response.

The group Doctors Without Borders has said the purported poison gas strikes near Damascus killed 355 people.

The U.N. chemical weapons experts conducted their first field testing in the western Damascus suburb of Moadamiyeh on Monday. They collected samples and testimony after a treacherous journey through government and rebel-held territory.