The UK and Russia remain at odds over the supply of arms to Syria's opposition, with Moscow suggesting that any move by the British government to give the rebels military equipment would breach international law.

The foreign secretary, William Hague, held talks in London with his Russian counterpart, Sergei Lavrov, in a meeting also attended by defence ministers Philip Hammond and Sergei Shoigu.

Last week Hague announced that Britain was stepping up its support for Syria's opposition National Coalition and would be providing non-lethal equipment to rebel fighters. The government is inching towards arming the rebels, with David Cameron indicating on Wednesday that the UK might be prepared to bypass an EU arms embargo as events on the ground unfold.

Speaking after talks with Hague, however, Lavrov said the supply of lethal weapons to the rebels would be illegal. "International law doesn't allow, doesn't permit, the supplies of arms to non-governmental actors. It's a violation of international law," he claimed. Lavrov also raised the spectre of western arms falling into the hands of radical Islamist groups such as Jabhat al-Nusra, placed on a terrorist list by the US.

Hammond responded: "We can't rule out anything in future. You can be sure that any action will be legal with a strong basis in international law."

Syrian rebels gather in an alley in Idlib. Photograph: AP

The differences between Moscow and the west over Syria are well known. The Kremlin has supplied large amounts of heavy weaponry to the regime of President Bashar al-Assad, including attack helicopters. The UK and other EU states have backed moderate democratic forces within the Syrian opposition. Hague has promised it "increased support". Russia and the UK were interested in a "political transition" in Syria, he said, though they disagree how to get there.

Wednesday's meeting amounts to a mini-thaw in relations between the Kremlin and London. It follows a series of rows over the 2006 polonium murder of the Russian intelligence officer Alexander Litvinenko and the government's failure, as Russia sees it, to extradite Russian nationals who have fled to the UK.

Andrei Lugovoi, a former KGB agent, said he would not take part in the inquest into Alexander Litvinenko's death. Photograph: Itar/Tass/Corbis

At a joint press conference, both foreign ministers dodged the Litivnenko issue. Hague alluded to the "one or two areas where we are known to have differences". Lavrov said on Moscow's extradition requests – repeatedly dismissed by British judges as politically motivated – there had been "slow progress".

On Tuesday Andrei Lugovoi, a former KGB agent accused of Litvinenko's murder, said he would not take part in an inquest being held into Litvinenko's death. Lugovoi said the hearing was "biased" following an application by Hague to keep secret the government's Litvinenko files. It emerged last year that at the time of his death Litvinenko had been working for the British and Spanish secret services, and was receiving regular payments from MI6. On Wednesday foreign office officials insisted Litvinenko and Russia's dismal human rights record was raised during "substantive" bilateral talks. Both sides appear keen to move on from the Litvinenko affair. But Hague's apparent attempt to reset relations with Russia over the past three years has attracted criticism. David Clark, chairman of the Russian Foundation, and a former aide to Robin Cook, said the gap between the two governments remained too wide for a meaningful breakthrough.

He said: "William Hague is now exploring options for providing military aid to the rebels and clearly envisages a future without President Assad as the only acceptable option, while the Russians are still determined to keep their ally in place. They have repeatedly blocked action on the UN security council that might compromise their objective." Clark described Hague as an "old-fashioned rightwing foreign policy realist". "He sees foreign policy in terms of trade and serious powers doing business with each other."

In Syria an EU adviser was killed on Tuesday in a rocket attack near the capital, Damascus, the 27-nation bloc confirmed.

Ahmad Shihadeh, a policy officer with the EU delegation in Syria, died on Tuesday in the Damascus suburb of Daraya, where he lived, according to the office of the EU's foreign policy chief, Lady Ashton.

A spokesman for Ashton said Shihadeh was Syrian, 32, and had worked for the EU for five years. It was the first death in the Syrian civil war of an EU employee. Shihadeh "died while providing humanitarian help to the community (of Daraya)", Ashton said. "Ahmad was known for his courage and selflessness."

Ashton also called for an end to the conflict: "As we approach the second anniversary of the uprising in Syria, I call again on all sides to take urgent steps to end the violence, which has led to the deaths of some 100,000 innocent citizens and over 1 million refugees seeking shelter in neighbouring countries."