US intelligence agencies are investigating accounts from European allies that Syria may have used chemical weapons in its war against opposition forces, a senior US official said Thursday.

There were indications a "deeply suspicious" agent may have been used in recent battles in Syria but spy services were still evaluating the information and had not reached a definitive conclusion, the official told AFP on condition of anonymity.

Chemical weapons may have been used in a limited, "localized" way and not on a large scale, the official said.

Diplomats had told reporters earlier this month that Western countries had "hard evidence" that chemical weapons had been used sporadically in Syria.

Britain and France have presented information to the United Nations on allegations that Syrian regime forces fired chemical arms in the city of Homs in December and at Ataybah near Damascus last month.

The Washington Post and Foreign Policy reported that London and Paris had informed UN Secretary-General Ban Ki-moon that soil samples, witness interviews and other information showed nerve agents were used in and around Aleppo, Homs and possibly Damascus.

The US official said the accounts from France and Britain were being taken seriously.

But other officials and analysts said intelligence agencies faced a difficult challenge trying to confirm the reports given the chaotic, violent conditions in Syria and a dearth of American spies in the field.

"The intelligence community has not verified the use of chemical weapons in Syria," said a second US official, who also asked not be named.

President Barack Obama has warned Damascus that the use of chemical weapons would constitute a "red line," implying but not explicitly threatening possible US military action.

If the allegations are confirmed, Obama -- who has sought to avoid any US military role in the civil war -- would face increased pressure to intervene.

The Obama administration has never precisely defined exactly what would constitute crossing the rubicon when it comes to chemical weapons in Syria.

The Pentagon announced earlier that it was bolstering a US military contingent in Jordan to prepare for worst-case scenarios, including securing chemical weapons in Syria.

The deployment of a US Army headquarters element to Amman will bring the total number of troops in Jordan to more than 200.

Top US military and intelligence officials previously clearly stated that Syria had not resorted to chemical weapons. But at congressional hearings this week, they sidestepped questions from senators on the sensitive subject.

Asked if President Bashar al-Assad's regime had crossed a limit in Syria by using chemical weapons, the Director of National Intelligence James Clapper told senators it was up to policy makers to define the "red line."

"That is a policy question and not one for intelligence to comment on," Clapper said.

A US intelligence official told AFP that political leaders "set the threshold" for action, while spy services "just tell them what we know."