LONDON (AFP) - Syrian opposition leaders renewed their appeals for arms at a meeting with US Secretary of State John Kerry on Wednesday, but a top jihadist group's pledge of loyalty to Al-Qaeda deepened Western concerns that weapons could fall into the wrong hands.

Mr Kerry and other G-8 foreign ministers held talks with members of the Syrian National Coalition, including opposition prime minister Ghassan Hitto, on the sidelines of a two-day ministerial meeting in London.

The US said it was mulling over ways to step up help for Syria's rebels, while Mr Kerry also met with Russian Foreign Minister Sergei Lavrov in a bid to find common ground on ending the conflict between the rebels and President Bashar-al-Assad.

Mr Kerry will meanwhile attend a "Friends of Syria" core group meeting on April 20 in Istanbul, a US official said.

But a statement on Wednesday by the head of Syria's jihadist Al-Nusra Front pledging allegiance to Al-Qaeda chief Ayman al-Zawahiri will only increase Western doubts about arming the rebels.

A top State Department official confirmed that, during a lunch hosted by British Foreign Secretary William Hague, the Syrian opposition leaders renewed appeals for lethal aid but Kerry "didn't promise anything".

The US and EU are currently providing non-lethal aid such as communications equipment, and are beginning to distribute food and medical supplies to the Free Syrian Army, but have stopped short of providing weaponry.

The announcement by the Al-Nusra front is likely to bolster assertions by Mr Assad's regime that it is fighting "terrorists" who want to impose an Islamic state.

"We are always considering a variety of options, we are going to continue to aid the opposition, working with them in terms of what they need, in terms of what we're willing to provide," the US official said.

Wednesday's talks had focused on ways of changing Mr Assad's calculations about the outcome of the conflict which is now in its third year and has cost some 70,000 lives, according to the UN.

"We need to have this continuing conversation which is why we are going back to Istanbul," the official said.

But he said the lunch with six Syrian opposition leaders was "a good, substantive discussion".

All sides emphasised "the importance of working together, the importance of them getting themselves more organised, which they said they were in the process of doing".

The Syria conflict was also "top of the agenda" as the G-8 foreign ministers met for dinner late Wednesday, Hague confirmed.

Meanwhile, a Syrian warplane dropped five shells on Wednesday on a rural area in north-eastern Lebanon near the Syrian border, a military official told AFP.

"A Syrian jet dropped five shells on the outskirts of Sarjal Ajram. No one was hurt," the official said on condition of anonymity.

The spiralling North Korean nuclear crisis and Iran's atomic ambitions were also discussed at Wednesday's meeting.

Mr Lavrov warned against heating up the Korean crisis with military manoeuvres, but stressed that Moscow and Washington had a common stand.

"On North Korea we have no differences with the United States," Mr Lavrov told journalists in Russian as he met Kerry.

"One just shouldn't scare anyone with military manoeuvres and there's a chance that everything will calm down," he added, without specifying which countries he believed were carrying out such military exercises.

On Iran - Syria's main ally in addition to Russia - the United States reacted with concern after Teheran this week unveiled a new uranium production facility and two extraction mines only days after talks with world powers on its disputed nuclear programme ended in deadlock.

"They have continued to move forward, we are very concerned about what they are doing," a senior State Department official said, asking to remain anonymous.

"We weren't blindsided about it, because we are rarely blindsided about the things that they are considering. But they did not specify that they were going to do this," the official said.

The G-8 groups Britain, Canada, France, Germany, Italy, Japan, Russia and the United States. Britain, which holds the rotating chairmanship of the group this year, will host a leaders' summit in Northern Ireland in June.