US Secretary of State John Kerry and other G8 foreign ministers are holding a second day of talks in London focused on Syria and North Korea.
Britain was expected to call for more help for the Syrian opposition but there were no signs of a major shift in policy, a day after rebels again appealed for weapons..
Western and Middle Eastern nations supporting the opposition will meet in Turkey on April 20, a US official said as foreign ministers of the Group of Eight nations met.
US Secretary of State John Kerry will attend the meeting of the so-called Friends of Syria "core group" in Istanbul, said
the official, who spoke to reporters on condition of anonymity.
North Korean threats of war will also be high on the agenda of the G8 talks, which began in London over dinner on Wednesday and were due to end on Thursday.
Iran's atomic ambitions, instability in north and west Africa, and climate change will also be up for discussion, according to Britain's Foreign Office.
The G8 consists of the US, Britain, France, Germany, Italy, Japan, Canada and Russia.
Leaders of the Syrian National Coalition (SNC) were present on the sidelines of the G8 meeting.
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".
My concern is that weapons that are delivered to Syria will then get into the hands of jihadists and terrorists which then could be deployed against moderate democratic forces, and I fear that some jihadist or Islamist terrorists see Damascus as a stopover to Jerusalem at best.
The United States, which on February 28 said it will for the first time give non-lethal aid to Syrian rebel fighters and more than double its aid to Syria's civilian opposition, has so far chosen not to provide arms to the rebels.
Hague, in a statement issued after the talks, said Britain was committed to finding a political solution to the crisis.
"We discussed what further assistance the UK could provide to save lives in Syria, and how we could work together to ensure this support was channelled most effectively," he said.
Before Thursday's morning session German Foreign Minister Guido Westerwelle reiterated that he is reluctant "when it comes to the topic of direct arms deliveries to Syria".
"To date I have seen no way to prevent these weapons getting into the wrong hands, namely those of radicals," he said. "My concern is that weapons that are delivered to Syria will then get into the hands of jihadists and terrorists which then could be deployed against moderate democratic forces, and I fear that some jihadist or Islamist terrorists see Damascus as a stopover to Jerusalem at best."
During a more than one-hour meeting with Russian Foreign Minister Sergei Lavrov on Wednesday, another US official said there was no sign of any change in Moscow's stance on Syria.
"It certainly didn't sound like they have changed their position a lot," said the official, who added that Kerry and Lavrov's talks on Syria were relatively brief.