British efforts to persuade the European Union to lift the arms embargo to Syrian rebels are facing defeat, with strong opposition from EU members alarmed that weapons could fall into the wrong hands.

The foreign secretary, William Hague, joined the French on Monday in arguing that supplying arms to "moderate" opposition forces would lead to less killing in Syria. Others argued the opposite, saying that arms supplies would only escalate the conflict.

Several countries – notably Austria, the Czech Republic, Finland, the Netherlands and Sweden – oppose it for fear that weapons may fall into the hands of Islamic extremist groups, such as Jabhat al-Nusra. Germany has been trying to fashion a compromise.

Guido Westerwelle, the German foreign minister, said he would try to "build bridges" at the meeting of EU foreign ministers, but consensus looked elusive, with participants delivering contradictory statements.

Both Westerwelle and Hague said the EU could fail to find a common position on the sanctions package that expires automatically on Friday unless there is agreement.

Hague added that if the negotiations collapsed, individual EU states would have to mount their own national sanctions against the Assad regime.

Britain and France have been pressing for a partial lifting of the arms embargo to the moderate sections of the Syrian opposition since November. Hague has argued that lifting the arms embargo would complement, rather than contradict, a peace process, since a militarily strengthened opposition could force President Bashar al-Assad to the negotiating table.

But Labour has questioned whether lifting the embargo would be legal or politically wise. "How would the government prevent British-supplied weapons falling into the wrong hands, and how does supplying weapons help to secure a lasting peace?" asked Douglas Alexander, the shadow foreign secretary.

"Syria today is awash with arms and in the House of Commons this week, MPs on all sides expressed real concern about the identity, intent and tactics of some of the rebel forces.

"In Washington, the prime minister clearly failed to convince President Obama of his case, so tomorrow in Brussels the UK's use of the veto would confirm that the prime minister had also failed to convince our European partners."

In Brussels, Turkey's foreign minister voiced strong support for the Anglo-French position. But last week in the US the White House strongly rebuffed Ankara for its hawkish position against Assad.

Oxfam's head of arms control, Anna Macdonald, said: "Allowing the EU arms embargo to end could have devastating consequences. There are no easy answers when trying to stop the bloodshed in Syria, but sending more arms and ammunition clearly isn't one of them.

"Transferring more weapons to Syria can only exacerbate a hellish scenario for civilians. If the UK and France are to live up to their own commitments – including those set out in the new arms trade treaty – they simply must not send weapons to Syria."

There were signs at the weekend that the Assad regime may attend an international peace conference in Geneva next month after the foreign minister, Walid al-Mouallem, said the meeting could present a "good opportunity for a political solution for the crisis in Syria". But he added: "No power on Earth can decide on the future of Syria. Only the Syrian people have the right to do so."

The French foreign minister, Laurent Fabius, is due to meet his Russian counterpart, Sergei Lavrov, and the US secretary of state, John Kerry, in Paris on Monday night to discuss the prospects for the meeting.

The UK-French attempt to lift the arms embargo has not been made any easier by the continued lack of unity within the rebel movement. Talks failed on Sunday to end a factional dispute over proposals to dilute Qatar's influence on rebel forces, with Saudi Arabia angling to play a greater role now that Iranian-backed Hezbollah is openly fighting for Assad.

Labour argued that the EU common position – an agreement that is usually a precursor to legislation – says member states must deny an export licence if there is a clear risk that the equipment may be used to commit violations of international humanitarian law or human rights.

The UN commission of inquiry in Syria reported in February that the rebels have committed war crimes, saying "war crimes, including murder, extrajudicial killings and torture, were perpetrated by anti-government armed groups".

UN security council resolution 2083 (2012) says all states shall take measures to prevent the direct or indirect supply of arms and related materiel to al-Qaida and other individuals and entities associated with them. Jabhat al-Nusra is linked with al-Qaida in Iraq.

A broader package of EU sanctions against Syria must in any case be renewed at this week's meeting, and if there is total deadlock on the related issue of an arms embargo the entire EU sanctions regime could collapse.

Many obstacles remain in the way of a peace conference taking place, including Russia's insistence that Iran be allowed to attend.

At the weekend Fabius said Iran's presence in Syria, through its officers who were "directing operations" and through its Hezbollah proxy, demonstrated that it had no place at the negotiating table.

"Yes, the Russians want Iran to take part in Geneva, but we're opposed because Iran is not after a political solution and, on the contrary, has thrown itself directly into that battle," he said.

Iran has denied it has forces in Syria supporting Assad's army, saying the accusations were invented by the "true enemies of Syria".