Secretary of State John Kerry hit back on Monday at the notion the United States is not doing enough to support the Syrian opposition.
"We are determined that the Syrian opposition is not going to be dangling in the wind wondering where the support is or if it is coming, and we are determined to change the calculation on the ground for President (Bashar al-Assad)," Kerrry said in London.
Making his first foreign trip as America's top diplomat, Kerry appeared at a news conference with British Foreign Secretary William Hague.
Kerry said the United States and its allies are pursuing a political resolution to the civil war that assures a broad cross section of Syrian society is represented in a new democratic government.
"We don't have the ingredients that bring about that dialogue at this point in time," he said before heading to his second stop of the trip in Berlin.
The comments came after the principal opposition group said it would not attend a meeting in Rome later this week of a group of more than 60 countries working to bring an end to the violence that has already claimed nearly 70,000 lives.
"Enough is enough. The whole world is not doing anything," Adib Shishakly, a member of the Syrian National Coalition, told CNN last week about the decision to not go to Rome. "We are not going to any more conferences."
Kerry will represent the United States at that meeting.
"I want our friends in the Syrian Opposition Council to know that we are not coming to Rome simply to talk," Kerry said on Monday. "We are coming to Rome to make a decision on next steps and perhaps even other options that may or may not be discussed further after that."
In advance of the meeting in Rome, Robert Ford, the U.S. ambassador to Syria, flew to Cairo on Sunday to meet with the Syrian Opposition Council to make the case for reversing its decision not to attend.
A senior State Department official said Kerry also spoke on the phone on Monday with Ahmed Moaz al-Khatib, the head of that group and encouraged him to go to Rome.
Soon after, the opposition council changed its position.
Al-Khatib said in a Facebook post that his group will now attend the conference after consultations with its members and others. The decision was also based on remarks by senior diplomats from the United States and Britain promising aid for Syrians and criticizing al-Assad's government.
Until recently, the loosely coordinated rebel movement was heavily outgunned by al-Assad's military forces, who leveraged superior weaponry and an air force.
But rebels have made significant gains recently. They control greater territory with the fighting moving closer to the capital with foreign countries funneling more weapons to the rebels, according to opposition fighters.
"Indeed, we procured new anti-aircraft and heavy defensive weapons donated from Arab and non-Arab countries recently," Louay Almokdad, political and media coordinator for the Free Syrian Army, told CNN on Sunday. "But they are not enough to stand against the regime's forces at this point."
The Obama administration has resisted providing arms to the rebel movement out of concern such weapons could wind up in the hands of extremist groups, such as the recently blacklisted Nusra Front.
Almokdad told CNN the Free Syrian Army recently met with representatives from unnamed Western governments "to discuss the dangers posed by Nusra Front," he said.
"We have assured the Western countries we met that if they give us weapons, then we will have complete control over those weapons and they will not fall into the hands of disorganized groups," he said.
The United States, the European Union and the Arab League have supplied non-lethal aid to rebels mainly in the form of communications equipment and humanitarian aid. In addition, the United States has pledged $365 million in humanitarian assistance for Syrians in the country and refugees who have fled.
The governments of Saudi Arabia and Qatar are among those thought to have also provided weapons to Syrian rebel groups.