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KIEV, Ukraine -- Ukraine's president rallied support Thursday for his plan to end fighting in the country's east in phone calls with the 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. He did not directly accuse Moscow of sending the tanks, but said it showed Russia had failed to fulfill promises to tighten border controls.
Russia has denied sending troops or weapons to Ukraine, describing Russian citizens who have joined the armed separatists as volunteers. There was no independent confirmation that the tanks had come from Russia.
In Washington, U.S. State Department spokeswoman Jen Psaki said if the military incursion was confirmed, it would be a "serious and disturbing escalation of the crisis in eastern Ukraine."
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. An AP reporter nearby heard the explosion and arrived to see a van in flames in front of the building. He saw three injured people being taken away.
The breakaway Donetsk People's Republic said on its Twitter feed that the van was used by one of the group's leaders, Denis Pushilin, 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 German Chancellor Angela Merkel, following a call the previous day with U.S. Vice President Joe Biden. 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 controlled by separatists near the village of Dyakove in eastern Ukraine. Three tanks went to the town of Snizhne, about 40 kilometers (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 control over the border.
U.S. Secretary of State John Kerry spoke Wednesday with Lavrov and urged him to encourage Putin to engage directly with Poroshenko, Psaki said.
"He also encouraged that conversation or engagement to focus on de-escalating the situation on the ground, and he called on Russia to halt the flow of militants and arms from Russia into eastern Ukraine, which is clearly relevant in this case," the spokeswoman said.
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 told reporters that 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 the resolution would concentrate on demanding fulfillment of proposals in the OSCE "road map" to resolve the conflict. It calls for non-violence, disarmament, national dialogue and elections.
Lavrov said Russia was not seeking authorization to send in peacekeeping troops. The Ukrainian rebels have suggested that Russia should send peacekeepers, but Moscow says that could only be done with U.N. 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, and its contacts or influence with the rebels are unclear.
"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.
Marko Drobnjakovic in Donetsk, Jim Heintz and Lynn Berry in Moscow, and Matthew Lee in Washington contributed to this report.