U.S. Secretary of State John Kerry is in Switzerland for talks with Russia's foreign minister on Syria and Ukraine.

Kerry arrived in Zurich early Wednesday and is hoping to resolve differences with Russia on who is eligible to join U.N.-mediated peace talks for Syria that are due to begin next week. Kerry will also be pushing for more progress on resolving conflict in eastern Ukraine between the government and Russia-backed rebels.

After seeing Sergey Lavrov, Kerry will go to Davos to attend the World Economic Forum and then travel on to Saudi Arabia, Laos, Cambodia and China.

Russia and Iran, which back Syrian President Bashar Assad, have severe differences with Saudi Arabia, other Arab states, the United States and Europe over which opposition groups should be considered terrorists and not allowed to be part of an 18-month political transition process that the U.N. has endorsed. One dispute is over the groups Ahrar-as-Sham and Jaish al-Islam, which Russia and Syria consider "terrorists" but Saudi Arabia, the United States and others view as legitimate opposition groups.

The dispute is threatening to delay the planned Jan. 25 start of U.N.-meditated peace talks.

"We're not unmindful of the fact that there still remains differences of opinion, and that this is a complicated process and that there is still quite a bit of work that needs to be done to get the meeting to occur," State Department spokesman John Kirby said Tuesday. "But it's our hope that this can continue to move forward, and that we can have this meeting on the 25th."

On Monday, U.N. chief Ban Ki-moon urged countries supporting opposing sides in the Syrian conflict to redouble efforts to reach agreement on the list of eligible opposition groups. Ban's appeal came as the U.N. special envoy for Syria, Staffan de Mistura, briefed the Security Council on his efforts to get the talks started and the leaders of Russia and Qatar met in Moscow to try to narrow their differences.

U.N. officials say they remain focused on starting the talks on Jan. 25 as planned, but they say they can't send out invitations until the key countries agree on an opposition list and have hinted at a possible delay.

In Washington, U.S. officials echoed those sentiments on Tuesday. One official said the talks had not yet been delayed, but that it was possible they could slip by a week or more.

In Moscow, meanwhile, a top Russian diplomat said he hoped the Lavrov-Kerry meeting would produce an agreement on the list.