"[The] Syrian people deserve better than the horrific violence that now invades and threatens their everyday lives -- the lives of innocent people, the lives of people who simply want an ability to have their government be accountable and to be able to be part of the governance of their own live," Kerry said during a news conference with British Foreign Secretary William Hague after a meeting.
Kerry said he and Hague condemned the "indiscriminate killing of innocent civilians" by President Bashar Assad's regime.
"And it is just further evidence that Assad has to go," he said.
Kerry thanked Hague for the British effort to "help dial up the pressure on the regime, for their contributions of humanitarian aid, and for hosting the transition conference last month."
Hague said he and Kerry would attend the Rome meeting of the Friends of Syria this week.
"An appalling injustice is being done to the people of Syria, which the world cannot ignore," Hague said.
Concerning Iran, Kerry said Iran having a nuclear weapon is "simply unacceptable."
The countries' two top diplomats discussed Iran's nuclear program a day before talks with Iran about its controversial program in Kazakhstan.
"And as we've repeatedly made clear, the window for a diplomatic solution simply cannot by definition remain open forever," he said. "But it is open today. It is open now. And there is still time, but there is only time if Iran makes the decision to come to the table and to negotiate in good faith."
London is Kerry's first stop on his first foreign trip as U.S. secretary of state. He is set to meet with other leaders in Germany, France, Italy, Turkey, Egypt, Saudi Arabia, the United Arab Emirates and Qatar.
Hague said they also discussed the importance of ending the Israeli-Palestinian conflict.
"The region and the world can't afford the current dangerous impasse in the peace process," Hague said, "for if we don't make progress very soon, then the two-state solution could become impossible to achieve."
During a question-and-answer session Kerry said the United States is taking allegations by Afghan President Hamid Karzai about torture and abuse in Wardak province seriously. Karzai Sunday told U.S. Special Forces stationed in Wardak they have two weeks to leave.
Kerry said the International Security Assistance Force is investigating and emphasized the U.S. has "listened very carefully" to Karzai's concerns.
Hague said the two agreed on the need to maintain a "robust, intelligent response to the threat from international terrorism."
"And finally ... we reiterate our commitment to a transatlantic trade agreement which would not only support jobs and growth in Europe and the U.S. but would be a much-needed boost to the world economy," Hague said.
"It is no secret that we both face economic challenges," Kerry said. "The fact is that Europe freestanding alone is the largest economy in the world. And when you join that together with the United States of America, we have a powerful ability to be able to affect the rules of the road and to be able to raise standards and, most importantly, create jobs for all of our people."