Sharply escalating tensions between Russia and Ukraine are raising concerns in the international community.
Kyiv has put its troops in the east and near Crimea on the highest state of alert.
The call for combat readiness from Ukrainian President Petro Poroshenko came as his ambassador to the UN said Russia had massed troops in Crimea and on the Ukrainian border.
The latest upsurge in tension follows Kremlin claims that Kyiv is using terrorist tactics to try to destabilise Crimea which was annexed by Russia from Ukraine in 2014 after a referendum not recognised internationally.
Moscow says it caught infiltrators after armed clashes on the border between Crimea and Ukraine over the weekend, and one of its soldiers and a Russian security service employee were killed. It has also presented explosive devices and ammunition it says were found.
Ukraine has rejected the accusations as false, saying they look like a pretext for Russia to escalate hostilities.
As the UN Security Council discussed the crisis on Thursday, Russian UN Ambassador Vitaly Churkin dismissed concerns about a Russian military build-up.
The US has urged restraint on both sides in what it is calling “a very dangerous situation”.
State Department spokeswoman Elizabeth Trudeau said on Thursday: “We believe that any actions, including rhetoric, including remarks, have the ability to escalate what is already a very tense situation and a very dangerous situation.”
Concern has also been expressed by an unnamed NATO official and by the German Foreign Office.
But Vladimir Putin has summoned his security council and, using some of his most aggressive rhetoric against Kiev since the height of the war two years ago, the Russian President has pledged to take counter-measures.
The belligerent posture heightened worries in Ukraine that Russia may plan to ramp up fighting in a war between Kyiv and pro-Russian eastern separatists that had been de-escalated by a shaky peace process.