Russia has warned Nato against sending any troops to help Ukraine, amid reports of a large Russian military build-up on its borders. Kremlin spokesman Dmitry Peskov said Russia would take "additional measures" if Nato were to make such a move. Sporadic, low-level clashes continue in eastern Ukraine between Ukrainian troops and Russian-backed separatists. US forces in Europe are now on higher alert, citing "escalations of Russian aggression" in the area. A Nato official told Reuters news agency that Russia was undermining efforts to reduce tensions in eastern Ukraine and Nato ambassadors had met on Thursday to discuss the situation. "Allies share their concerns about Russia's recent large-scale military activities in and around Ukraine," the official said. Ukraine's President Volodymyr Zelensky joined the criticism, saying "military exercises and possible provocations along the border are traditional Russian games". In a phone call with Mr Zelensky on Friday, US President Joe Biden "affirmed the United States' unwavering support for Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression in the Donbas and Crimea", the White House said in a statement. Meanwhile, Mr Peskov accused Ukrainian forces of staging "provocations" on the frontline in eastern Ukraine, where a fragile ceasefire is violated daily. Ukraine's Foreign Minister Dmytro Kuleba told the BBC that there had been a build-up of Russian military at the northern border between Ukraine and Russia, along the eastern border and "also in the illegally occupied Crimea". "So from three directions we observed a military build-up of the Russian federation," he said, adding: "Ukraine is not looking for any escalation - we do not need war." In the worst flare-up in recent months, four Ukrainian soldiers died in separatist shelling on 26 March near Shuma, a village in the Donetsk region. There have been only low-level incidents since then.