US Secretary of State John Kerry flew into the Lithuanian capital Vilnius late Friday for talks with EU ministers aimed at shoring up support for US strikes on Syria.
In his 14th trip in seven months since becoming America's top diplomat, Kerry will also travel to Paris and London and meet Arab leaders, including Palestinian president Mahmud Abbas.
Apart from talks with Lithuanian leaders on Saturday, Kerry will also hold informal talks with EU foreign ministers, State Department spokeswoman Jen Psaki said.
They will discuss "the Middle East, including Syria, Egypt, and the ongoing direct, final status negotiations between the Israelis and Palestinians," Psaki added.
Kerry said his aim in Vilnius was to "continue to lay out the evidence we have collected and seek to broaden support for a limited military response to deter the Assad regime from launching another chemical weapons attack."
In an opinion piece Friday published in the Huffington Post, he insisted: "The costs of inaction here are much greater than the costs of action."
Later Saturday, Kerry will fly on to Paris to meet French officials.
While British deputies voted against taking part in any military response, France's administration has swung behind punitive US military strikes against Syria.
US President Barack Obama called for the action after a suspected chemical weapons attack last month on a Damascus suburb allegedly killed more than 1,400 people.
Kerry, who after serving as a naval lieutenant in Vietnam became a passionate anti-war advocate, sought to explain how he could now lead the US charge for strikes on Syria.
"The answer is, I spoke my conscience in 1971 and I'm speaking my conscience now in 2013," he wrote in the Huffington Post.
"Make no mistake: If another Vietnam or another Iraq were on the table in the situation room, I wouldn't be sitting at the witness table before Congress advocating for action.
"I spent two years of my life working to stop the war in Vietnam, and made enemies and lost friends because of my decision to speak my mind," he wrote.
"So I don't come to my view on the use of military force anywhere without real reflection. I do so with an eye towards facts and reason."
On Sunday, still in Paris, Kerry will also meet Arab League leaders to update them on the Syria issue and on progress in the Israeli-Palestinian peace process.
Huge divisions emerged over the Syrian crisis at a G20 summit in St Petersburg, Russia, attended by Obama and which ended Friday.
On Thursday, Washington accused Moscow of holding "hostage" the UN Security Council.
Russia hit back on Friday, warning the United States against targeting Syria's chemical arsenal. Its foreign ministry said in a statement that "such actions would represent a dangerous new turn in the tragic development of the Syria crisis."
Kerry will wrap up his whirlwind tour with talks in London late Sunday with Abbas.
The two men have met numerous times over the past months as the American diplomat worked to kickstart direct talks with the Israelis after a three-year stalemate.
A senior US official said Kerry would also meet with Israel Prime Minister Benjamin Netanyahu "in the near future." Netanyahu is expected to travel to the United States later this month for the annual UN General Assembly.
Direct talks between Israel and the Palestinians resumed on July 29, after Kerry shuttled between Jerusalem, the West Bank and Amman for several months seeking to push both sides back to the negotiating table.
The two sides have since met three times in August and in early September in Jerusalem.
In line with Kerry's desire to keep the details of the negotiations secret in order to give the process a chance to work, little has leaked about the talks.
But Palestinian officials have complained about the lack of direct US involvement, even though Kerry has appointed veteran diplomat Martin Indyk to act as the US go-between to the talks.
Kerry is due to fly back to the United States on Monday.