Al-Qaida-linked groups have attempted to obtain chemical weapons for use in Syria, British Prime Minister David Cameron said Friday.

"We haven't seen any credible reporting of chemical weapons use by the Syrian opposition," he told reporters at Downing Street.

"However, we assess that elements affiliated to al-Qaida in the region have attempted to acquire chemical weapons for probable use in Syria."

He said he believed that the use of chemical weapons in Syria was "sanctioned and ordered by the Assad regime." Britain had not yet made a decision on whether to arm moderate rebels, he said.

Earlier on Friday, Western diplomats said that the United States is considering imposing a no-fly zone in Syria, its first direct military intervention of the two-year-old civil war, after the White House said Syria had crossed a "red line" by using nerve gas.

After months of equivocating, President Barack Obama's administration said on Thursday it would now arm rebels, having obtained proof the Syrian government used chemical weapons against fighters trying to overthrow President Bashar Assad.

Two senior Western diplomats said Washington is mulling a no-fly zone close to Syria's southern border with Jordan.

"Washington is considering a no-fly zone to help Assad's opponents," one diplomat said. He said it would be limited "time-wise and area-wise, possibly near the Jordanian border," without giving details.

France said on Friday that the no-fly zone was unlikely for now because of opposition from some members of the United Nations Security Council.