Kerry was due to fly to London to brief Washington's European allies after “brainstorming” talks in Lausanne with the main players in Syria's bloody five-year-old conflict.
The Swiss meeting did not produce a concrete plan to restore the truce that collapsed last month amid bitter recriminations between Washington and Moscow and new fighting on the ground.
But Kerry insisted the new, leaner contact group had come up with some plausible ideas that would be fleshed out in the coming days and might lead to a new, stronger ceasefire.
“The way it wrapped up was to have several ideas that need to be quickly followed up,” he said after talks with Russia, Iran, Iraq, Qatar, Jordan, Saudi Arabia, Egypt and Turkey.
“The next contact on trying to follow up on this is going to be immediately, because this is urgent, and we're not letting any grass grow under our feet.”
But he said it was too early to reveal what the ideas were, and that high-level contacts — but not a ministerial-level meeting — would continue on Monday to develop them.
He was expected, however, to raise the issues with Britain's Foreign Minister Boris Johnson and senior European colleagues, after flying to London later on Sunday.
Britain, France, Germany and Italy are members of the International Syria Support Group and have met before with other countries interested in resolving the Syrian crisis.
But US officials now say the full group is too unwieldy to make rapid decisions, and that Saturday's Lausanne meeting was more productive for being focused on the main regional players.
The US envoy's tone was upbeat, but diplomats from all sides warned against hopes of a rapid ceasefire.
And away from the talks, Moscow's actions showed no sign that it might be softening its strong support for Syrian President Bashar al-Assad and his campaign against US-backed rebels.
Fierce fighting was also continuing elsewhere in the multi-front conflict, with Turkish-backed fighters closing in on Dabiq, a symbolic stronghold of the Islamic State group. And in Aleppo, Assad's Russian-backed government forces intensified their bombardment of the rebel-held east of the city, further damaging any prospect of a renewed ceasefire.
Kerry and Russian Foreign Minister Sergei Lavrov, once joint sponsors of international peace efforts, met ahead of the broader talks, but US officials insisted that their “bilateral track” remained dead.
Lavrov joined Kerry in welcoming the idea of bringing other powers into the mix saying, “we must prolong our contacts over the coming days”.
President Barack Obama has been adamant that US forces will not become caught up in the war and Kerry was hoping that talks with Russia and regional powers may yield new ideas. The talks come as Moscow faces growing criticism over its backing for Assad's assault in divided Aleppo.
Air strikes hit rebel-held parts of Aleppo again Saturday, according to the Syrian Observatory for Human Rights, a British-based watchdog with a network of sources on the ground.
Against this bloody backdrop, a leading opposition group slammed the talks, saying they would not stop the killing Abdal Ahad Stefo, deputy head of the Istanbul-based National Coalition, told AFP the negotiations “will only lead to wasting more time… and the shedding of more Syrian blood”.
Aleppo has been engulfed by some of the worst violence of the conflict since the collapse of last month's truce deal.
Kerry and Lavrov were joined by UN Syria envoy Staffan de Mistura, as well as top diplomats from Turkey, Saudi Arabia and Qatar — all nations that back Syrian opposition forces.
Iran, a key Assad supporter, was being represented by Foreign Minister Mohammad Javad Zarif, who sat opposite Foreign Minister Adel al-Jubeir of Saudi Arabia, Tehran's foe.
Secretary of State John Kerry said lower level officials would be in contact again as early as Monday to flesh out a new agenda, but refused to reveal details of the initiatives.
Kerry described the meeting as a “brainstorming” session and that despite tensions between the parties had not descended into rancour, with all agreeing on the urgency of the crisis.
“I would characterise this as exactly what we wanted,” he told reporters at the Lausanne hotel where he met Russian, Iranian, Saudi, Turkish, Egyptian, Jordanian, Iraqi and Qatari envoys.
Kerry welcomed what he said was “a very candid, first time discussion, as open and free-wheeling as this, with all of the key parties at the table simultaneously.”
“There were some difficult moments, where there was obviously tension, but everybody was constructive, he said, of a meeting that included traditional foes Tehran and Riyadh,” he said.
After the Lausanne meeting, Kerry was to fly on to London on Sunday to brief his European allies on the new diplomatic path, which will focus on a smaller number of main players.