The Syrian opposition has dropped its boycott of talks with John Kerry, William Hague and other western backers in Rome this week after promises of substantially increased aid, western diplomatic sources have said.

The leader of the opposition National Coalition, Moaz al-Khatib, informed the Italian government on Monday afternoon that he had persuaded his partners to attend a "Friends of Syria" meeting in the Italian capital, reversing an announcement on Sunday that the group would stay away from all such talks because of the international community's failure to end the bloodshed.

The move came after Kerry, the US secretary of state, phoned Khatib urging him to attend.

Kerry, in London on his first foreign trip since becoming secretary of state, said he was sensitive to the frustration of the Syrian opposition, but promised new American support for the National Coalition would "come to maturity by the time we meet in Rome". Other US measures would be discussed in the Italian capital if the opposition took part in the Friends of Syria meeting, he said.

Kerry insisted US policy remained the pursuit of a political resolution, suggesting that direct military aid was not on the immediate agenda, but he added: "We are determined that the Syrian opposition is not going to be dangling in the wind, wondering where the support is, if it is coming."

Khatib said: "We will reconsider the decision on the boycott in light of the strong message of support we and Syrian people got over the weekend." He had been under fire from some in the coalition for his offer to open talks with the Damascus regime before the Syrian president, Bashar al-Assad, steps aside.

In Moscow, the Syrian foreign minister, Walid al-Moualem, said that the government was prepared to hold talks with the rebels, while pursuing its fight "against terrorism". Rebel commanders have said that they will not enter talks while government forces keep up their bombardment of rebel-held towns.

The British foreign secretary urged the opposition to stay involved in talks, promising a large boost in support.

"An appalling injustice is being done to the people of Syria, which the world cannot ignore," Hague said. "That is why in the United Kingdom, we believe we must significantly increase our support for the Syrian opposition, on top of our large contributions to the humanitarian relief effort, and we are preparing to do just that.

"In the face of such murder and threat of instability, our policy cannot remain static as the weeks go by, and it is an important opportunity in Rome on Thursday to discuss this with our allies and partners."