William Hague has condemned the "completely unacceptable" sight of tanks entering eastern Ukraine from Russia as he renewed calls on Moscow to act to reduce tensions or face further sanctions.
Pro-Russian rebel leaders have declined to say where they obtained the tanks, which were part of a column of armoured vehicles that crossed the border yesterday.
Ukrainian troops drove separatists from occupied buildings in the southern port of Mariupol, the second-largest city in the Donetsk region which rebels have declared independent of Kiev.
"The entry into the sovereign territory of Ukraine of three Russian tanks is completely unacceptable," Mr Hague said.
"Russia needs to commit to defusing tension. This means securing borders, withdrawing all military forces, and preventing further violence in eastern Ukraine.
"I urge the Russian leadership to work with President Poroshenko to restore stability and to regain the trust of the international community.
"The Russian leadership should be clear that the international community stands ready to impose further sanctions if Moscow continues to provoke instability and does nothing to stop further violence."