Opposition leaders had protested what they see as inaction by other nations in the face of violence from Syrian President Bashar Assad's regime.
Kerry not only made a public plea at a joint news conference Monday with British Foreign Secretary William Hague, he also called Moaz Khatib, leader of the Syrian Opposition Council, "to encourage him to come to Rome," a senior U.S. official said. The official, who spoke on condition of anonymity because he wasn't authorized to speak publicly on the matter, described the conversation as "good" but declined to offer more detail.
Spokesman Walid al-Bunni said the council had decided to send a delegation to Rome after all.
Al-Bunni told Al-Arabiya TV the decision was made based on guarantees al-Khatib heard from western diplomats that the conference would be different and that the opposition would receive real commitments this time. "We will go and we will see if the promises are different this time," he said.
After speaking with Khatib, Kerry flew to Berlin from London, the first stop of his first trip as secretary of state a hectic nine-country dash through Europe and the Middle East.
Kerry had also dispatched his top Syrian envoy to Cairo in hopes of convincing opposition leaders that their participation is critical to addressing questions from potential donors and securing additional aid from the United States and Europe.
"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," Kerry told reporters in London after meeting British Prime Minister David Cameron and Hague. "We are not going to let the Syrian opposition not have its ability to have its voice properly heard in this process."
Vice President Joe Biden also reached out Monday in a phone call to Khatib. Biden commended him and other members of the Syrian Opposition Coalition for their courage. He also emphasized President Barack Obama's commitment to a post-Assad government that protects the rights of all its citizens, according to a description of the call provided by the White House.
For his part, Hague said the violence in Syria, especially recent scud missile attacks on the city of Aleppo, was unacceptable and that the west's current position could not be sustained while an "appalling injustice" is being done to Syrian citizens.
"In the face of such murder and threat of instability, our policy cannot stay static as the weeks go by," Hague told reporters, standing beside Kerry. "We must significantly increase support for the Syrian opposition. We are preparing to do just that."
"We are not coming to Rome simply to talk, we are coming to Rome to talk about next steps," Kerry said, adding that he was sympathetic to opposition complaints that they were not getting the support they need to defend themselves against the Assad regime or oust him from power.
"I am very sensitive to that frustration," recalling that as a U.S. senator he was one of several who pushed the administration to consider military aid to the Syrian opposition.
"But I am the new secretary of state ... and the president of the United States has sent me here and sent me to this series of meetings and in Rome because he is concerned about the course of events.
"This moment is ripe for us to be considering what more we can do," he said, adding that if the opposition wants results, "join us" in Rome.
Meanwhile, Syrian Foreign Minister Walid al-Moallem said Monday the Assad regime was ready to hold talks with opposition leaders, the first time that a high-ranking Syrian official has stated publicly that the government would meet with the opposition. Al-Moallem made his comments after meeting in Moscow with Russian officials.
In Washington, State Department spokesman Patrick Ventrell said Moallem's remarks appeared positive but expressed caution about the seriousness of the offer.
"I don't know their motivations, other than to say they continue to rain down horrific attacks on their own people," Ventrell told reporters. "So that speaks pretty loudly and clearly."
If the Assad regime is serious, he said, it should inform the U.N. peace envoy, Lakhdar Brahimi of its readiness for talks. Ventrell said the Assad regime hasn't yet done that.
Obama administration officials have debated whether the U.S. should arm the rebels, with Defense Secretary Leon Panetta and Joint Chiefs of Staff Chairman Gen. Martin Dempsey having said they urged such a course of action. The White House has been unwilling to do so for fears the weapons could end up in the wrong hands. Currently, the U.S. provides only non-lethal support and humanitarian aid.
The United Nations says at least 70,000 people have been killed in Syria's 2-year civil war, which began as an uprising against Assad's regime.
Kerry said the Syrian people "deserve better" than the violence currently gripping their country as he stood alongside Britain's Foreign Secretary William Hague.
Associated Press writers Cassandra Vinograd in London and Bradley Klapper in Washington contributed to this report.