* There may be no Security Council vote on Syria -diplomats
* U.S.: No point waiting for U.N. given Russian opposition
* Syria denies carrying out any chemical attacks (Adds State Department reaction, diplomats)
UNITED NATIONS, Aug 28 (Reuters) - Syria asked the United Nations on Wednesday to order chemical experts in Damascus to investigate three rebel attacks in which he said Syrian troops "inhaled poisonous gas," while Britain pushed for the Security Council to act on the crisis.
The United States dismissed the appeal by Syria's U.N. envoy, saying it had refused chemical experts access to Syria to investigate allegations poisonous gas had been used repeatedly in the country's 2-1/2-year-old civil war.
The United Nations has received at least 14 reports of possible chemical weapons use in Syria. After months of diplomatic wrangling, a team of experts, led by Swedish scientist Ake Sellstrom, arrived in Syria on Aug. 18.
The U.N. team was initially going to look into three incidents, but its priority became investigating an alleged gas attack in the rebel-held suburbs of Damascus last week, which activists say killed hundreds of civilians.
Assad's government denies responsibility for the attack, and, like Russia, has suggested the rebels may be responsible.
Syrian U.N. Ambassador Bashar Ja'afari said he has written to U.N. Secretary-General Ban Ki-moon to ask that Sellstrom's team also "investigate three heinous incidents that took place in the countryside of Damascus on (August) 22nd, 24th and 25th where members of the Syrian army inhaled poisonous gas."
The U.N. investigators are due to leave Syria this weekend and U.N. spokesman Farhan Haq told reporters that the world body had not asked the Syrian government for an extension to the 14-day visit.
"The team has the ability to investigate other incidents as needed," Haq said, adding that the initial three incidents they were due to examine when they arrived in Syria would be investigated "in due course."
Western powers are planning possible military action in Syria in order to punish Syrian President Bashar al-Assad's government, who they blame for last week's attack.
"The Americans want to go quickly," said a western diplomat.
U.S. State Department spokeswoman Marie Harf was asked by reporters in Washington if she thought the call for a new U.N. investigation was intended to stall for time to delay an attack. U.N. diplomats say any Western airstrikes on Syria are unlikely while the chemical experts are in the country.
"I don't want to venture to guess why they're doing what they're doing, but I think it's clear that we will not allow them to hide behind a U.N. investigation into the use of chemical weapons to prevent any response from the United States," Harf said.
The Syrian government and the opposition have accused each other of using chemical weapons, and both have denied doing so. The U.N. inquiry is trying to establish only whether chemical weapons were used, not who used them.
Ja'afari's request to Ban on Wednesday came as the five permanent veto-wielding members of the U.N. Security Council - Russia, China, the United States, Britain and France - met to discuss a British draft resolution that would condemn Assad's government for carrying out the attack last week.
The draft resolution would authorize "all necessary force" to protect civilians from chemical weapons - giving approval for military action in Syria by western powers.
The meeting of the five Security Council members lasted about an hour and all of the envoys declined to comment afterward. Moscow said earlier that Britain was "premature" in seeking a resolution to protect Syrian civilians.
Russia, Syria's main arms supplier, as well as China, have already vetoed three resolutions condemning Assad.
U.N. diplomats said the Security Council may not even vote on the resolution since Russia has made clear it opposes any military intervention in Syria. Harf said Washington saw no point in waiting for U.N. Security Council action given Moscow's position on Syria.
"We cannot be held up in responding by Russia's ... continued intransigence at the United Nations," Harf said. "Quite frankly, the situation is so serious that it demands a response." (Additional reporting by Arshad Mohammed and Paul Eckert in Washington; Editing by Jackie Frank and Stacey Joyce)