Ukraine’s parliament has voted to support the president’s motion to impose martial law for 30 days.
The move will include a partial mobilisation and strengthening of the country’s air defence.
The measures before parliament also included vaguely worded steps such as “strengthening” anti-terrorism measures and “information security”.
President Petro Poroshenko had asked legislators to back his motion to impose martial law after Sunday’s incident near Russia-occupied Crimea when Russian borders guards opened fire on three Ukrainian navy vessels before seizing the ships and their crews.
Parliament voted overwhelmingly for the martial law motion after a five-hour debate.
“We consider it as an act of aggression against our state and a very serious threat,” Mr Poroshenko said.
“Unfortunately, there are no ‘red lines’ for the Russian Federation.”
The Ukrainian navy said six sailors were wounded when Russian coastguards opened fire late on Sunday on three Ukrainian ships near the Kerch Strait and then seized them.
Russia said three Ukrainian sailors were lightly injured and given medical assistance.
An emergency meeting of the United Nations Security Council was called for Monday, while the European Union and Nato called for restraint from both sides.
Mr Poroshenko had a phone call with Nato secretary-general Jens Stoltenberg to discuss the situation, and Nato later said that, at Mr Poroshenko’s request, its ambassadors and Ukraine’s envoy will hold emergency talks in Brussels.
Nato said Mr Stoltenberg expressed the US-led military alliance’s “full support for Ukraine’s territorial integrity and sovereignty, including its full navigational rights in its territorial waters under international law”.
European Union chief Donald Tusk called on Russia to stop provoking Ukraine and urged the authorities to return any Ukrainian ships or sailors it is holding.