the opposition to revoke its boycott on a Friends of Syria conference in Rome on Thursday after an appeal at a joint press conference in London.
Syrian National Coalition chief Ahmed Moaz al-Khatib said on his Facebook page his group would attend after Kerry and Hague "promised specific aid to alleviate the suffering of our people".
The Coalition said on Saturday it was withdrawing from the 11-nation meeting and planned visits to Washington and Moscow in protest at the world's silence over the mounting civilian death toll in Syria.
But Kerry told them: "I want our friends in the Syrian opposition council to know that we are not coming to Rome simply to talk.
"We are coming to Rome to make a decision on next steps," he said.
Syrian foreign minister Walid al-Muallem said in Moscow on Monday that the authorities in Damascus were ready to talk to armed rebels.
"We are ready for dialogue with all who want dialogue, including those who are carrying arms," he said, the first time a senior official of Syrian President Bashar al-Assad's regime has made such a proposal.
"We still believe in a peaceful solution to the Syrian problem," said Muallem, proposing the creation of a government coalition that would negotiate with both the "external and internal opposition."
But the rebel Free Syrian Army's chief of staff Selim Idriss dismissed Muallem's offer.
"I am not going to sit down with him (Assad) or with any other member of his clique before all the killing stops, or before the army withdraws from the cities," Idriss told pan-Arab broadcaster Al-Arabiya.
Kerry flew into Berlin for talks on Tuesday with Russia's foreign minister Sergei Lavrov to try to agree on a way to end the crisis, over which the two countries are deeply divided.
Earlier in London -- the first stage of a nine-nation tour of US allies in Europe -- Kerry too gave a cool response to Muallem's offer.
"It seems to me that it's pretty hard to understand how, when you see these Scuds falling on the innocent people of Aleppo, it's possible to take their notion that they're ready to have a dialogue very seriously," he said.
The Britain-based Syrian Observatory for Human Rights says 58 people, including 36 children, were killed in a Scud missile strike Friday on the northern city of Aleppo.
Syrian information minister Omran al-Zohbi on Monday denied firing Scud missiles against rebels, in an interview with Arabic-language Russia Today.
Muallem was in Russia for talks with Lavrov, whose country is one of the few major powers to maintain ties with the Syrian government.
Rebels have been fighting the Assad regime since an uprising against his rule in March 2011 and now control large parts of the country, especially in the north. According to the UN, the fighting has claimed 70,000 lives.
Lavrov renewed Russia's call for rebels and the regime to hold direct talks to end the conflict, warning that pushing for military victory risked destroying the country.
'No sign of a political breakthrough': Hague
After the talks with Kerry, Britain's Hague also expressed "frustration" at the situation and said: "There has been no sign of a political and diplomatic breakthrough."
With the death toll rising, policy on Syria needed to change and that would be the subject of the Rome talks, he added.
Britain wants to provide more support for the Syrian rebels but is bound by an EU arms embargo, which last week European foreign ministers decided not to lift.
The Syrian Observatory for Human Rights said at least 30 Syrian troops and 23 rebels were killed over the past 24 hours in fierce clashes for control of a police academy in the northern Aleppo province.
It also reported that rebels shot down a regime helicopter near a military base elsewhere in the north of the province as insurgents pressed on with attacks on the police academy in the west.
And a suicide car bomb killed five soldiers at a checkpoint in northeast Damascus late Monday, the Observatory reported.
The Britain-based Observatory, which relies on a vast network of activists on the ground and medics, said at least 92 people were killed in violence across Syria on Monday.
A UN staffer from the Golan Heights observer force that monitors a ceasefire between Syria and Israel has gone missing, the United Nations said Monday.