President Barack Obama has called on Congress to approve up to 1 billion dollars to support eastern European countries, which he says "will be a powerful demonstration of America's unshakable commitment to our NATO allies".
On the first of a four-day European trip, Obama promised support in the face of Russia's actions in Ukraine, which includes "rotating additional ground troops and F-16 aircraft into Poland".
Poland and some of its NATO neighbours have been asking for extra troops for months. However, a big increase in US forces could prompt Moscow to take similar actions.
Speaking in a news conference in Warsaw he said that while maintaining sanctions against Russia, the US still wants good relations with the Kremlin.
Nato defence ministers will discuss temporarily reinforcing forces in Poland, a spokesman for the German defence ministry said.
The spokesman said the 28-member alliance would consider reinforcing its Multinational Corps Northeast in the Polish town of Szczecin, in an attempt to reassure countries nervous about Russia's intervention in Ukraine.
Nato defence ministers will meet in Brussels on Tuesday to review measures the alliance had taken given Russia's actions towards Ukraine.
France has announced it will send four Rafale fighter jets to Poland next week as part of a demonstration of Nato's commitment to central Europe and the Baltic States to assuage concerns after the escalation of the situation in Ukraine.
The French defence ministry said that the four warplanes will be sent to Malbork in northern Poland on 28 April on a double mission: to train the Polish air force and stand ready for air patrols over the Baltic states under Nato command.
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.