* Russian and British officials meet to improve ties
* Differences over Syria highlight disunity
* Russia warns of radical Islamists (Adds Lavrov, Hague, EU's Ashton quotes, details)
By Mohammed Abbas
LONDON, March 13 (Reuters) - Britain and Russia tried to present a unified front over the Syrian crisis on Wednesday, but their deep differences on whether outside powers should arm the rebels were quickly laid bare.
Russian Foreign Minister Sergei Lavrov said arming Syria's opposition would be against international law, a day after Britain said it might bypass a European Union arms embargo to do just that, and accused the rebel side of failing to provide negotiators to find a political solution to the conflict.
"International law does not permit the supply of arms to non-governmental actors and our point of view is that it is a violation of international law," Lavrov told a news conference in London via a translator.
The meeting of Lavrov, British foreign minister William Hague and the British and Russian defence ministers was aimed at improving the often frosty ties between the two countries.
Hague reiterated his wish for U.N. Security Council-backed measures against President Bashar al-Assad, but said he had not reached agreement on the issue with Lavrov, whose country has repeatedly blocked such action in the past.
Hague repeated that he did not rule out arming Syria's outgunned rebels, a stance apparently backed by France, which on Tuesday said it was difficult to deny the rebels arms while Syria's government was being supplied with weapons.
Russia has been a major supplier of weapons to the Syria state, and insists Assad's departure should not be a precondition of any peace talks, as demanded by rebels.
"We've never ruled out anything in the future, we don't know how grave this situation will become," Hague said.
"Anything that we do and any development in our policy ... will be legal and will be clearly stated to our country and to the international community."
Lavrov insisted the only way to solve the crisis was through dialogue, and pointed to a U.N. communique issued in Geneva last year which called for the formation of a transitional governing body and for all Syrian factions to engage in talks.
"The opposition has not yet accepted the communique as the foundation for the negotiations. The regime has said that they shaped a committee for negotiations ... the opposition has not shaped such a team yet," Lavrov said.
"DANGEROUS AND EFFECTIVE" ISLAMISTS
European Union foreign policy chief Catherine Ashton said on Wednesday no EU government had proposed lifting the bloc's arms embargo to arm the opposition.
The embargo is part of a package of EU sanctions on Syria that roll over every three months, with the last extension unanimously agreed by the EU last month. Without another agreement to renew or amend the ban, the embargo lapses.
Germany has warned that arming the rebels could destabilise other countries in the volatile region.
Lavrov said the "most dangerous and effective" anti-Assad group in Syria was the al Qaeda-linked al-Nusra Front.
"The Americans have included this group in the list of terrorist organisations, and this decision ... provoked indignation in one of the National Coalition members of Syria. Let us just keep that in mind when we are discussing Syria," he said.
The two-year-old conflict started out as a pro-democracy movement, but has descended into an increasingly sectarian war. Some 70,000 people have been killed and more than one million refugees have fled the violence. (Additional reporting by Adrian Croft and Justyna Pawlak in Brussels; editing by Andrew Roche)