KIEV — 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 had 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 he said it showed that Russia had failed to fulfill its 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, State Department spokeswoman Jen Psaki said that 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 eastern city of Donetsk, where the separatists have taken over a regional administration building. A reporter heard the explosion and arrived at the scene 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 that he was not in the vehicle. The same tweet said four people were injured, one of whom was in grave condition.
President Petro Poroshenko of Ukraine, who took office less than a week ago, told President Vladimir Putin of Russia that it was unacceptable that tanks had crossed the border, Poroshenko’s spokesman, Svyatoslav Tsegolko, said. A Kremlin statement said Poroshenko told Putin about his plan for resolving the crisis but did not say whether the two discussed the tanks.
The Ukrainian president also spoke Thursday with Chancellor Angela Merkel of Germany, following a call the previous day with Vice President Joe Biden of the United States. Poroshenko has said that he is willing to negotiate, but not with those he calls terrorists, and that he 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 25 miles from Dyakove, where one remained, while the two others headed toward the town of Horlivka, where they were engaged by the Ukrainian military, Avakov 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.
Secretary of State John Kerry spoke Wednesday with Foreign Minister Sergey Lavrov of Russia, urging 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,’’ she said.
Russia’s UN ambassador said Thursday that he intends to introduce a Security Council resolution aimed at stopping the violence in Ukraine. Vitaly Churkin told reporters that the resolution will focus on political efforts being carried out by the Organization for Security and Cooperation in Europe, ‘‘so far not successfully.’’
Lavrov said earlier Thursday the resolution would concentrate on demanding fulfillment of proposals in the security 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 that Russia should send peacekeepers, but Russian officials say that could be done only with UN 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.