Kerry said he has every expectation that both sides in the conflict, which has left more than 70,000 dead, will participate in an international conference to negotiate a peaceful transition in Syria. The conference, which Kerry said likely will be held in early June, is noteworthy because it will be endorsed by both the U.S. and Russia, which are on opposite sides of the Syrian conflict.
Speaking in Stockholm, the Swedish capital, the top American diplomat rejected suggestions that officials from Assad's regime will refuse to attend the conference. Earlier Tuesday, Syria's information minister said the regime wants more details about the proposed initiative before it decides whether to attend.
"If he decides not to come to the table it will be another one of President Assad's gross miscalculations. Now, I don't believe that that is the case at this moment," Kerry told reporters in Stockholm.
He added: "If President Assad decides to miscalculate again about that, as he has miscalculated about his own country's future over the course of the last years, it is clear the opposition will be receiving additional support, there will be additional efforts made and unfortunately the violence will not end."
Kerry did not specify what form that additional support might take.
The U.S. has committed to providing $510 million in humanitarian assistance to Syria. The White House is considering arming the opposition forces — and has already provided the Syrian rebels with some military equipment like body armor. But U.S. officials say no decision on arming the rebels will occur until the peace talks happen. They say the administration will gauge if the negotiations can plot a path toward a political transition and if the process fails, the U.S. could then resort to arming the rebels. The officials spoke on condition of anonymity because they weren't authorized to speak publicly on the matter.
President Barack Obama has said he does not foresee any circumstance where he would order U.S. ground troops into Syria, even as Washington pursues more evidence about the Assad regime's purported use of chemical weapons.
Kerry was in Stockholm on his way to a meeting of the eight-member Arctic Council in the city of Kiruna in northern Sweden. He said Assad's government already has provided names of potential negotiators to Russian officials — a strong signal that the regime is planning to attend the conference. Opposition officials also are planning to attend, Kerry said.
"I keep hearing some people suggest somehow that the process is moving away, not closer," Kerry said. "I just don't agree with that. Enormous plans are being laid."
He said the talks would focus on ensuring that all of Syria's people would be protected in a potential cease-fire.
"If Assad decides not to come, the world will see how empty his rhetoric is as well as his intent," Kerry said.
On another battle front, Kerry said Obama will "very shortly" announce how many U.S. troops will remain in Afghanistan after 2014, when the White House has pledged to end the war more than a decade after invading.
Kerry would not hint how many troops would remain, but said they would continue to train and equip the Afghan army and fight against counterterror threats to ensure the country does not return to its days as a safe haven for violent extremists.
"I can guarantee you, it will be enough to get the mission accomplished that the president has defined," Kerry said.
Associated Press writers Bradley Klapper contributed to this report.
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Copyright 2013 The Associated Press.