Explosions have been reported near Kiev - just minutes after Russia's president Vladimir Putin authorised a military operation in eastern Ukraine.
Lights have gone off in some parts of Kiev.
Reports also suggest blasts have been heard in cities including Odesa and Kharkiv.
Putin made the announcement in a TV address just before 6am Moscow time.
Putin's announcement was made at the same time as a UN Security Council meeting was taking place - urging peace.
Speaking afterwards, secretary general Antonio Guterres appealed directly to the Russian leader.
"In the name of humanity, bring your troops back to Russia.
"In the name of humanity, do not allow to start in Europe the worst war since the beginning of the century."
EU leaders will meet in Brussels this evening for an emergency summit over the Ukraine crisis.
Last night the Taoiseach told his parliamentary party what we're witnessing now's like nothing seen since World War 2.
European Affairs Minister, Thomas Byrne, says today's summit of EU leaders is hugely important.
"This is a very dangerous situation, I think people in that region are quite frankly frightened at the moment.
"The European Union will be showing leadership to show Putin we mean business - sanctions are going to be our weapon of choice in the defence of democracy."
