The Group of Seven major economies has agreed to "move swiftly" on new sanctions against Russia because of Russia's actions in Ukraine.
In a joint statement released late Friday, the G-7 nations — the U.S., Canada, Britain, France, Gernamy, Italy and Japan — said they would take measures to intensify "targeted sanctions."
A U.S. official said the sanctions could begin as early as Monday.
Ukrainian Prime Minister Arseniy Yatsenyuk has accused Russia of wanting to occupy Ukraine "militarily and politically," as both Kyiv and Moscow mass troops close to their mutual border. Yatsenyuk told an interim Cabinet meeting that Moscow "wants to start World War Three."
Meanwhile, U.S. military officials said Friday Russian aircraft flew into Ukrainian airspace several times in the previous 24 hours.
A Defense Department spokesman called on Moscow "to take immediate steps to de-escalate the situation." The official gave no details on where the incursions took place.
The flights come as Russia increases military exercises along the Ukrainian border.
On Thursday, the top U.S. military officer, Chairman of the Joint Chiefs of Staff General Martin Dempsey, spoke with his Russian counterpart.
Pro-Russian separatists in eastern Ukraine detained a team of international military observers on Friday. The separatists seized a bus carrying more than a dozen people from the Vienna-based Organization for the Security and Cooperation in Europe near the town of Slovyansk.
Russia's envoy to OSCE said Saturday Russia will take all possible measures to free the observers. Andrei Kelin told Russian news agencies the OSCE team needed "to be freed as soon as possible."
Slovyansk's self-declared mayor, Vyacheslav Ponomarev, said the group was detained because a Ukrainian military official was traveling with them.
"The sign OSCE does not mean protection for an officer of the (army) General Headquarters. We found an employee of the army headquarters. After an investigation we will decide what we are to do."
The OSCE wrote on Twitter that it had lost contact with the German-led monitoring team.
U.S. State Department spokeswoman Jen Psaki condemned the detentions, calling the tactic repressive and cowardly.
"We are deeply concerned about reports that unidentified gunmen have abducted a Vienna document inspection team - that's of course part of the OSCE - and their Ukrainian escorts in the town of Slovyansk. The team was reportedly taken to the administrative building which is being held by armed pro-Russian separatists."
Pro-Russian militants are occupying government buildings in around a dozen cities in eastern Ukraine.