Poland's Foreign Minister Radoslaw Sikorski has called Russia's actions in eastern Ukraine an "aggression" that has created the most serious security crisis in Europe in decades.
"This for sure is an aggression," Sikorski told state radio. "Poland's position will be worked out in the next couple of hours."
He said the need for "strong words" had passed and called for "actions of the whole international community".
The European Court of Human Rights has found that Poland violated the EU Convention on Human Rights by allowing the CIA to detain two al-Qaeda suspects on Polish territory.
The case was brought by two Saudi-born men who said they were flown in secret to a CIA-run jail in a Polish forest and subjected to treatment that amounted to torture.
Polish officials have denied the existence of the CIA jail. The US has acknowledged that it kept al-Qaeda suspects in facilities outside the US, but has not said where.
The EU court ruled that Poland violated articles around the prohibition of torture, the right to liberty and the right to an effective investigation of their allegations.
Poland has been ordered to pay the men 230,000 euros.
A plane which crashed in Poland had been operated by a private parachuting school, took off from the Rudniki airfield just outside Czestochowa, a fire spokesman said.
"From what we know 11 people were killed," said Justyna Sochacka, spokeswoman for the Air Rescue said
"There were 12 people onboard the plane."
A spokesman for the headquarters of fire services, Pawel Fratczak, said the crashed plane was a Piper PA-31 Navajo.
"The plane caught fire after it hit the ground," Fratczak said. The Piper PA-31 Navajo plane is produced by US firm Piper Aircraft.
An emergency medical services and fire services spokesman said eleven people were feared dead after a plane taking them on a parachute jump crashed in southern Poland shortly after take off.
One man in his forties survived and was taken to hospital in the city of Czestochowa in serious condition, 207 kilometres (129 miles) south-west of Warsaw.
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