Syria crisis: US insists Assad can have no future role John Kerry's carefully worded statment may have been designed to win over Syrian opposition figures. Photograph: Franco Origlia/Getty Images International efforts to resolve the Syrian crisis reached new levels of uncertainty on Thursday when John Kerry, the US secretary of state, insisted that Bashar al-Assad could not be part of a future transitional government - casting doubt on a US-Russian agreement to convene a conference of the regime and opposition. Kerry's comments were at odds with the view of Sergei Lavrov, his Russian counterpart, that President Assad's departure could not be a precondition for the peace talks. The unresolved question of what role the Syrian leader could play has been at the heart of faltering diplomatic moves to end the conflict since a UN meeting in Geneva last June. Speaking in Rome, Kerry said the parties were working to "effect a transition government by mutual consent of both sides, which clearly means that in our judgement President Assad will not be a component of that transitional government." David Cameron is to discuss Syria when he meets Vladimir Putin in Moscow on Friday, British officials said. The UK position remains that Assad "has no place" in a future Syria, while arguing for a political solution. But Britain and France are also continuing efforts to lift or amend the EU arms embargo on Syria to allow them to supply arms to the rebels. The US, already helping coordinate weapons deliveries by Gulf states, is also considering supplying arms. International alarm about Syria has been fuelled in recent days by the alleged use of chemical weapons, last weekend's Israeli air raids near Damascus and a sense that a crisis that has already cost a reported 70,000 lives and created huge refugee flows that are destabilising neighbouring countries, shows no sign of winding down. In another development, the leader of Lebanon's Hezbollah, Hassan Nasrallah - a key Assad ally - said on Thursday that Syria would step up its support of the militia group. "If the aim ... was to prevent the strengthening of the resistance's capabilities, then Syria will give the resistance sophisticated weapons the like of which it hasn't seen before," he warned in a televised address. Syrian opposition figures said they remained sceptical about Tuesday's US-Russian initiative, with some accusing Washington of reneging on its previous insistence that Assad must go. Kerry's carefully worded statement seemed designed to persuade them to attend the proposed conference despite their profound misgivings. The US ambassador to Syria, Robert Ford, met the Syrian opposition coalition in Istanbul on Wednesday. Some of its leaders fear a ploy to persuade unrepresentative opposition figures to enter talks with the regime. Washington may be deliberately employing ambiguous language about Assad's fate, perhaps considering a formula under which the Syrian president might agree to resign later. Assad has, however, shown no sign of readiness to step down. Syria's government said, meanwhile, that it welcomed the US-Russian statement but reserved the right to fight terrorism - its phrase for all opposition to the regime. It is likely to insist on an end to arms supplies as a condition for attending any conference. Kerry said after talks with the Jordanian foreign minister, Nasser Judeh, that he hoped Russia would not supply advanced ground-to-air missile systems to Syria - following a report in the Wall Street Journal that Israel has told the US that such a deal was imminent. France's foreign minister Laurent Fabius said in an interview with Le Monde that the UN should follow the US in declaring the Islamist Jabhat al-Nusra a terrorist organisation to differentiate it from other Syrian rebel groups - apparently implying ones affiliated to the Free Syrian Army that France, Britain and others may be preparing to support actively if current diplomatic efforts fail. The Nusra front, which has links to al-Qaida, is emerging as the most effective force fighting the Syrian government, FSA commanders say.