LONDON  - Britain is "increasingly concerned" about the use of chemical weapons in the Syrian conflict, Foreign Secretary William Hague said on Monday, warning that "the world is watching".

Speaking in the House of Commons, Hague also said Britain and France would continue to press for a lifting of the EU embargo blocking the delivery of weapons to the Syrian opposition despite one rebel group's pledge of allegiance to Al-Qaeda.

"The UK is increasingly concerned that there is evidence of the use of chemical weapons in Syria. These allegations must be fully and urgently investigated," Hague told lawmakers during an update on last week's G8 ministerial meeting in London.

"We welcome the UN secretary general's announcement of an investigation into the allegations and again call on the Syrian regime to cooperate fully and allow the investigation unfettered access to all areas.

"They should take heed that the world is watching and those who order the use of chemical weapons or participate in their use must be held to account."

Hague refused to comment on media reports that British military scientists who have analysed soil samples smuggled out of Syria have found forensic evidence that chemical weapons have been used.

Addressing the issue of support for the opposition forces fighting President Bashar al-Assad's regime, Hague stressed that Britain had taken no decision to arm the rebels but continued to work with France to lift the EU embargo. "The UK and France argue that we will need further amendments to the EU arms embargo or even to lift it altogether," Hague said.

"As things stand we need greater flexibility if we decide urgent action is necessary, for example in response to a specific incident, or continued grave deterioration on the ground, or to create the conditions for a successful political transition."

He acknowledged concerns that weapons could fall into the wrong hands, which were amplified by last week's announcement by the Al-Nusra Front that it was pledging allegiance to Al-Qaeda chief Ayman al-Zawahiri.

Meanwhile, Syria's air force on Monday carried out several air strikes on rebel enclaves in and around Damascus, while fresh clashes between troops and rebels raged to the east of the capital, a monitoring group said.

"At least one civilian was killed in an air strike on Qaboon" in northeast Damascus, the Syrian Observatory for Human Rights said, "while regime troops pounded the district of Jubar" in the east. Amateur video distributed by the Syrian Revolution General Commission (SRGC), a network of activists on the ground, showed a cloud of grey smoke rising after the air raid. The regime also used warplanes to bombard the rebel-held towns of Yabroud, Douma and Harasta east of Damascus, as well as Sbeineh southwest of the capital, said the Observatory.

Meanwhile, "fierce clashes raged on the edges of Jubar (in eastern Damascus), near Abbasiyeen square," said the group.

Rebels view the square as a strategic target because it lies well within the confines of the capital, the regime's main bastion of power.

The SRGC, meanwhile, reported the army's use of heavy artillery fire on Yarmuk in southern Damascus, with tanks also striking other rebel enclaves nearby. Violence rages daily in southern Damascus, with rebels holding the outer edges of several districts, while the regime tries to push them out.

Germany on Monday airlifted more than 30 badly wounded Syrian refugees from Jordan for medical care in its hospitals in what it called a "humanitarian gesture" amid the bloody civil war.

 The injured, among them women and children, were flown aboard a specially equipped air force plane and were to be transferred to four German military hospitals in cities including Berlin and Hamburg.

"The civil war in Syria has already claimed far too many lives," said Westerwelle at a press conference with Arab League chief Nabil al-Arabi.

"We want to make a small contribution to lessen the unimaginable suffering of the people of Syria, and we want to show that Germany at this time of hardship is especially close to the Syrian people."

Syrian opposition chief Ahmed Moaz al-Khatib had requested the help - believed to be the first medical airlift of its kind by a European nation amid the more than two-year-old Syrian conflict.

Westerwelle refused to be drawn on whether any rebel fighters were among the wounded, saying that the 36 patients were selected strictly according to medical, not political criteria.

"If someone is lying on the operating table and must be saved, then the doctor doesn't ask, 'how were you injured?', but 'how can we help?' - and that's how Germany is looking at it," said Westerwelle.

He called the medical airlift "a humanitarian gesture" that was meant to strengthen the status and authority of the moderate opposition forces, which were a "visible, credible alternative to the Assad regime."

A foreign ministry spokesman said Germany might organise similar airlifts in future, but that no further flights were planned as yet.

Last month Germany said it was ready to take in another 5,000 Syrian refugees in coming months, raising the total to about 13,000 since early 2012.

The conflict in Syria, which started as a political uprising against the regime of President Bashar al-Assad, has now claimed some 70,000 lives, according to the United Nations.