The talks were set to begin in Geneva, Switzerland, on Monday, but they were delayed because of ongoing discussions about who should represent the opposition, the U.N.'s special envoy for Syria, Staffan de Mistura, told reporters.
"The position of the United States is and hasn't changed; that we are still supporting the opposition politically, financially and militarily," Kerry said, when asked about the opposition reports. "I don't know where this is coming from. Maybe it's a pressure thing or maybe it's an internal political thing. Maybe it's a -- I don't know, but that is not the situation."
Kerry said he remained confident that talks would take place, and soon. He stressed the decision would be made by de Mistura.
"I hope it is possible -- I would express it that way -- to resolve whatever rumors are being circulated by one -- there are a lot of factions here, folks, and when -- lot of different interests at play too. So I just don't buy into this public back-and-forth. It doesn't serve any purpose," Kerry said.
But Khaled Khoja, president of Syrian National Coalition, accused Kerry of offering nothing new and of taking the line similar to that of Russia and Iran when Kerry met with Riyad Hijab, the general coordinator of the High Negotiations Committee, and other HNC delegates representing the Syrian opposition in the upcoming negotiations in Riyadh on Saturday.
"Kerry did not make any promises, nor did he put forward any initiatives. He has long been delivering messages similar to those drafted by Iran and Russia which call for the establishment of a 'national government' and allowing (Syrian President) Bashar al-Assad to stay in power and stand for re-election," Khoja said.
He added: "The core of (the) issue is not related to setting a date for negotiations, but whether there is an international political will to create an appropriate environment for negotiations. The moment there is such an environment, we will be ready to engage in negotiations as we have already assembled our delegation negotiators."
Any ceasefire won't stop ISIS
At a news conference in Geneva, de Mistura said the invitation list was nearly finalized and invitations would be sent Tuesday.
The talks, which would push for a nationwide ceasefire agreement among all factions other than ISIS and al Nusra Front, would now begin on Friday, he said.
Stakeholders had expressed "very different opinions" about who should be at the table, he said.
"Therefore we have been careful and extremely thorough in wanting to make sure that when and if we start, we start on the right foot," de Mistura said.
His mandate was to involve "the broadest possible spectrum of the opposition" to al-Assad's government, he said, adding that it was important that women and representatives from civil society had strong representation in the peace talks.
However, armed groups that the U.N. Security Council has designated terror organizations -- ISIS and al Nusra Front -- would not being included in the talks or be part of any ceasefire agreement, he said.
"It's clear by all those that are attending ... and also the Security Council meeting, that the suspension of fighting regarding ISIL in particular, and al Nusra is not on the table," he said, using the acronym for the Islamic State of Iraq and the Levant. "But there is plenty of other suspensions of fighting that can take place."
Talks would focus on enforcing a broad ceasefire and opening the way for increased humanitarian aid to affected populations in the country, ravaged by nearly five years of a war that the U.N. says has cost more than 300,000 lives.
The Syrian conflict started when protesters took to the streets demanding Assad's ouster. When his regime repressed the protests, clashes escalated into a civil war, gradually becoming a bloody free-for-all as the opposition, government troops and ISIS extremists fight for dominance.
U.S., Russia urge talks
Earlier, Kerry and his Russian counterpart had urged de Mistura to announce a date soon for the start of talks, Russia's Foreign Ministry said in a statement.
The call by Kerry and Russian Foreign Minister Sergey Lavrov came after the two spoke by phone Monday, the ministry said.
Kerry said the gathering of top stakeholders in Geneva will provide more insight on the way forward.
"We are trying to make absolutely certain that when they start that everybody is clear about roles and what is happening, so you don't go there and wind up with a question mark or a failure," he said.
"You don't want to start Day One not being able to make progress."
Kerry said he also spoke to de Mistura and the foreign ministers of Saudi Arabia, France and Turkey.
Their conversations were aimed at reaching a consensus on how a planned ceasefire would proceed.
"You have to get to the negotiations without preconditions and have to get to the ceasefire and lay down the road ahead for the transition discussion itself," he said.
"It's not in our hands," Kerry said. "If they are not serious, war will continue."
'What does it mean they are winning?'
Kerry slammed reports that Syria believes it is winning because of military action in the nation by its close ally, Russia, and that as a result, Syria may not be as willing to make concessions.
"What does it mean they are winning? They control a tiny part of the country. Most is controlled by ISIL or by the Kurds or by someone else," Kerry said.
"That is not the Russian attitude, I will tell you that."
Kerry said it will become clear "very quickly, in a month or two or three" whether Syria, Russia and Iran are serious about negotiating.
Efforts to negotiate an end to the conflict have been hindered by strong opposition to the Assad regime, which is perceived as brutal, and by the fractured nature of the opposition, which includes ISIS as well as more moderate groups supported by the United States.
In addition to the hundreds of thousands killed, more than 4 million people have fled the country, many of them headed for Europe.
Kerry said world powers of the International Syria Support Group are tentatively scheduled to meet February 11 to address any problems that arise from the talks.