Nato has said an emergency meeting will be held on Ukraine at the request of Poland as it felt under threat from the neighbouring crisis.
Nato's Secretary General tweeted:
Poland made the request under Article 4 of the Washington Treaty, meaning that "any ally can request consultations whenever, in theopinion of any of them, their territorial integrity, political independence or security is threatened."
The seizure of government buildings by armed men in Ukraine's Crimea region could lead to a regional conflict, the Polish foreign minister has said.
Radoslaw Sikorski said the move was "a drastic step and I'm warning those who did this and those who allowed them to do this, because this is how regional conflicts begin. This is a very dangerous game."
The German defence minister has also expressed concerns over the events on the Crimea peninsula, saying the breakup of Ukraine must be avoided.
Countries in Eastern Europe have issued their support for Ukraine, after new parliamentary speaker Oleksander Turchinov, an ally of former prime minister Yulia Tymoshenko, was granted presidential powers:
We welcome the ongoing democratic change in Ukraine, Yuliya Tymoshenko release. We remain firmly committed to Ukraine's European integration
Foreign Secretary William Hague has spoken to Poland's Foreign Minister Radoslaw Sikorski about the ongoing situation in Ukraine:
The Polish foreign minister told ITV News today that martial law was a real possibility for Ukraine before the peace deal was agreed.Read the full story ›
ITV News Europe Editor James Mates talks to Polish foreign minister Radoslaw Sikorski after the diplomat was overheard telling a Ukrainian opposition leader: "If you don't support this [deal] you'll have martial law, you'll have the army. You will all be dead."
The Polish foreign minister Radoslaw Sikorski was filmed by ITV News telling a Ukranian protest leader: "If you don't support this [deal] you'll have martial law, you'll have the army. You will all be dead.
Following the encounter, the foreign minister spoke with ITV News' Europe Editor James Mates:
Q: You were overheard warning them of the possibility of martial law if they don't accept the deal. Is that a serious threat do you think?
Sikorski: It was a real possibility.
Q: That the army could have been sent in?
Sikorski: To my knowledge the interior troops were being readied.
Q: They've stepped back from the brink is what you're saying?
Sikorski: Well, as you can see it's almost miraculous. Within minutes of the agreement being signed the riot police are leaving.
ITV News Europe Editor James Mates is in Kiev:
Polish FM @sikorskiradek confirms to me threat of martial law was real "To my knowledge interior ministry troops were being readied"
The Polish foreign minister has been filmed telling a Ukranian protest leader: "If you don't support this [deal] you'll have martial law, you'll have the army. You will all be dead."
When asked by ITV News Europe Editor James Mates if he had managed to convince the opposition, Radoslaw Sikorski appeared to reply: "I don't know."
The ministers later returned for further negotiations, after which they announced they had backing from the opposition.
This update has been edited to include video subtitles
The Polish Prime Minister has said a draft agreement with Ukraine's Presidency is ready, but that a final deal has not yet been reached.
Donald Tusk said he would be cautious in predicting the end of the Ukraine crisis.