Ukrainian President Volodymyr Zelenskyy has signed a law allowing reservists to be called up for military service without a mobilization first being announced, his office said Wednesday.
The move comes amid a vast Russian military buildup near its border with Ukraine and a surge in cease-fire violations in eastern Ukraine.
On Tuesday EU foreign affairs chief Josep Borrell said some 100,000 Russian troops were now stationed at the border, making it Russia's largest ever buildup there.
The new law was passed by the Ukrainian parliament in late March. The President's office said it would allow the government "to quickly replenish the military units of defense forces, which will significantly increase their combat capability during the rapidly growing military aggression".
In exceptional circumstances, Ukrainian citizens who have concluded contracts in the military reserves could also be conscripted on the orders of the Supreme Commander-in-Chief of the Armed Forces.
Ukrainian Foreign Minister Dmytro Kuleba said on Tuesday that the Russian buildup along the border was “expected to reach a combined force of over 120,000 troops” in about a week.
He urged the West to impose stricter sanctions against Moscow by targeting entire sectors of the Russian economy.
Russia has rejected Ukrainian and Western concerns about the buildup and insisted it forms part of military readiness drills in response to "threatening" actions by the US and NATO.
At the same time, the Kremlin has warned Ukrainian authorities against trying to use force to retake control of the rebel east, saying that Russia could be forced to intervene to protect civilians in the region.
Zelenskyy had also said in a video address on Tuesday that Ukraine "would never start a war, but would always stand until the end” - and offered a proposal to Russian President Vladimir Putin to meet in eastern Ukraine to defuse tensions.
Kremlin spokesman Dmitry Peskov on Wednesday said he was not sure Putin had had the time to look at Zelenskyy's address.