KIEV, Ukraine -- Ukraine's president rallied support Thursday for his plan to end fighting in the country's east in phone calls with U.S., Russian and German leaders, even as he condemned what Ukrainian officials called an incursion of armored vehicles from Russia.

The Ukrainian interior minister said three tanks crossed into Ukraine along with other armored vehicles from Russia and were attacked by military forces fighting pro-Moscow separatists.

Russia has denied sending troops or equipment into Ukraine, describing Russian citizens who have joined the armed separatists as volunteers.

There was no independent confirmation that the tanks were from Russia.

The reported incursion followed statements earlier Thursday by Russia's foreign minister that the separatists were ready for a cease-fire but that Kiev had to initiate the process.

Late Thursday, an explosion shook the center of the major eastern city of Donetsk, where the rebels have taken over a regional administration building.

The breakaway Donetsk People's Republic said on its Twitter feed that a van used by one of the group's leaders, Denis Pushilin, was damaged but said he was not in the vehicle.

The same tweet said four people were injured and one was in grave condition.

Ukrainian President Petro Poroshenko, who took office less than a week ago, told Russian President Vladimir Putin that it was "unacceptable" that tanks had crossed the border, according to his spokesman, Svyatoslav Tsegolko.

A Kremlin statement said Poroshenko told Putin about his plan for resolving the crisis in the east, but did not say whether they discussed the tanks.

The Ukrainian president also spoke Thursday with U.S. Vice President Joe Biden and German Chancellor Angela Merkel.

Poroshenko has said he is willing to negotiate, but not with what he calls terrorists, and could offer amnesty to those who don't have "blood on their hands."

Interior Minister Arsen Avakov said a "column" with armored vehicles had crossed from Russia through border control points manned by separatists near the village of Dyakove in eastern Ukraine.

Three tanks went to the town of Snizhne, about 25 miles from Dyakove, and one remained there while the two others headed toward the town of Horlivka and were engaged by the Ukrainian military, he said. He added that part of the column was destroyed.

Avakov said the incursion had been going on for three days and took place despite Russian statements of interest in a peaceful solution and promises to increase border controls.

Russia's U.N. ambassador said Thursday that he intends to introduce a U.N. Security Council resolution aimed at stopping the violence in Ukraine.

Vitaly Churkin said it will focus on political efforts being carried out by the Organization for Security and Cooperation in Europe, "so far not successfully."

Russian Foreign Minister Sergey Lavrov said earlier Thursday that the resolution would concentrate on demanding fulfillment of proposals in the organization's "road map" to resolve the conflict.

It calls for nonviolence, disarmament, national dialogue and elections.

Lavrov said Russia was not seeking authorization to send in peacekeeping troops. The Ukrainian rebels have suggested Russia should send peacekeepers, but Moscow has said that could be done only with United Nations authorization.

Tensions between Russia and Ukraine sharpened in February after pro-Russian President Viktor Yanukovych was driven from office by a mass protest movement among people who wanted closer ties with Europe.

Ukraine's government and Western countries allege that Russia is fomenting or supporting the uprising in the eastern part of the country where Russian speakers are more numerous.

Insurgents have declared two regions independent and are seeking annexation by Russia.

Moscow denies it has agents in eastern Ukraine.

"We know that the rebels in the southeast are ready to hold fire, but the first step by all rights should be made by the Kiev authorities," Lavrov said.

The Organization for Security and Cooperation in Europe's secretary-general on Thursday visited a camp in Russia for refugees from eastern Ukraine and called for the fighting to end.

According to the Interfax news agency, Lamberto Zannier said the conflict in the east "is craziness which must be stopped."

Russia has said some 30,000 people have fled the fighting.

Poroshenko this week called for establishing safe-passage corridors for civilians who want to escape the violence, but there have been no visible steps toward creating them.

A Section on 06/13/2014