(Adds details, Security Council meeting, U.S. involvement)
UNITED NATIONS, Aug 21 (Reuters) - The United States, Britain and France will officially ask U.N. Secretary-General Ban Ki-moon on Wednesday to open a U.N. investigation into an alleged gas attack in Syria, U.N. diplomats said on condition of anonymity.
Western and regional countries have publicly called for U.N. chemical weapons investigators, who arrived in Damascus three days ago to look into previous allegations of such attacks, to be dispatched to the scene of what may be one of the deadliest incidents of Syria's two-year-old civil war.
The U.N. mission of Argentina, the president of the 15-nation U.N. Security Council this month, said the council would hold an emergency meeting at 3:00 p.m. EDT (1900 GMT) to discuss the latest alleged use of chemical weapons in Syria.
"Today, we are formally requesting that the United Nations urgently investigate this new allegation," White House Deputy Press Secretary Josh Earnest said in a statement. "The U.N. investigative team, which is currently in Syria, is prepared to do so, and that is consistent with its purpose and mandate."
In addition to the United States, Britain and France, Russia's Foreign Ministry called for a fair and professional investigation into reports that troops loyal to Syrian President Bashar al-Assad were responsible for the attack.
But Moscow suggested that rebels could have staged the assault to provoke international action.
Syria's opposition accused Assad's forces of gassing many hundreds of people - by one report as many as 1,300 - in the attack. Assad's government denied using chemical weapons. (Reporting by Michelle Nichols and Louis Charbonneau; Editing by Vicki Allen)