Kiev, Ukraine (CNN) -- U.S. President Barack Obama threatened Russia on Friday with new sanctions over its actions in Ukraine as the Ukrainian Prime Minister also came out with tough words against Moscow.
The United States could announce further sanctions as early as next week, U.S. officials say.
Russian attempts to start conflict in Ukraine will lead to military conflict in Europe, Prime Minister Arseniy Yatsenyuk warned.
He also charged Russia with aiming to occupy his country "militarily and politically," and said Moscow wants to start a third world war.
His strong statement came amid a sharp escalation in tensions.
Russia's defense chief ordered new military drills Thursday near the border with eastern Ukraine after Ukrainian forces said they killed five pro-Russian militants in an operation to clear roadblocks near the city of Slavyansk.
Serhiy Pashinskiy, chief of staff to acting Ukrainian President Oleksandr Turchynov, said Friday that the drills had now entered its "second stage," aimed at encircling Slavyansk and cutting off additional supplies or support.
And on Friday afternoon, Pentagon spokesman Col. Steve Warren said Russian aircraft entered Ukrainian airspace on several occasions over the previous 24 hours. He urged Russia "to take immediate steps to de-escalate the situation."
Yatsenyuk said he'd asked Russia to explain the presence of its troops on Ukraine's border within 48 hours -- but 24 hours later, Kiev is still waiting for an official response.
At the same time, he said, the world is lined up with Kiev in its efforts to restore stability and preserve "the system of world security that Russia wants to destroy."
Just as Kiev announced the second phase of its so called "anti-terrorism operation," behind the barricades of the security services building, at the command and control center, the pro-Russian forces are confident they can rebel any sort of attack.
"We have 2,000 fighters at the ready and plenty of weapons," says Evgeniy Gorbik, a former military man turned spokesman for pro-Russian militants.
On Friday, Ukraine's Interior Ministry reported that 13 people on board a bus -- including foreign observers and military officials -- were detained at the entrance of Slavyansk. The ministry said the captors were unknown, but that the detainees were taken to a security office that is under the control of pro-Russian separatists.
The detainees are seven European representatives from theOrganization for Security and Co-operation, five Ukrainian military representatives and a bus driver, the ministry said.
"We strongly condemn the fact that the members of the Organization for Security and Co-operation mission were taken hostage by pro-Russian separatist forces, this is completely unacceptable," Ukrainian Deputy Foreign Minister Danylo Lubkivsky told CNN.
Negotiations were ongoing to secure their release, the ministry reported.
"We also demand and call upon Russian authorities to exercise all their influence to release the hostages. We count on the solidarity between American and European parties in deescalating the situation and stop the aggressor. That's our principle stance, we would like to avoid any casualties, any victims and any conflicts," added Lubkivsky.