The first American troops arrived in Poland Wednesday, after Washington said it was sending a force of 600 to the Baltic states as the crisis over Ukraine deepens.

Some 150 soldiers from the 173rd Airborne Brigade touched down early afternoon in Swidwin, in the northeast of the country, according to an AFP journalist at the scene.

Pope Francis on Saturday called on Catholics to bring the message of God "to the very ends of the earth" during an Easter vigil mass in St Peter's Basilica.

Francis said the core of the Easter message was about returning to the basics of faith and asking: "Have I gone off on roads and paths which made me forget it?"

Politics overshadowed Easter observances in Kiev and Moscow, with the Orthodox religious leaders in each capital trading barbs over the crisis in Ukraine -- while the U.S. reportedly prepared to send ground forces to neighboring Poland.

Patriarch Filaret thundered to the faithful in pro-West Kiev that Russia was an "enemy" whose "attack" on Ukraine was doomed to failure because it was evil and contrary to God's will.

Poland's defense minister has said U.S. ground forces will be sent to his country to expand NATO's presence there as events unfold in neighboring Ukraine, The Washington Post reported.

Tomasz Siemoniak told the Post Friday that details were already being worked out and that Poland would play a leading regional role "under U.S. patronage" in the plan expected to be announced next week.

Imperial Tobacco on Tuesday announced the closure of factories in Britain and France with the loss of 900 jobs, citing declining sales in Europe, tougher anti-smoking measures and the growth of contraband sales.

The two factories concerned are at Carquefou, near Nantes in western France, where 327 people are employed, and  Nottingham in central England, which has 540 staff.

Third countries should stay out of the debate over Ukraine's future constitution, Poland's prime minister said Wednesday, amid Russia's insistence on solving the crisis through federalization.

"Ukraine's future constitution should, in our opinion, be adopted by Ukrainians alone. And the shape of Ukraine's constitution should not be a topic of discussion for third countries," Prime Minister Donald Tusk told reporters.

France will offer to send four fighter jets to the Baltic states and Poland to boost NATO air patrols over the region, a source close to the French defense minister said Friday.

Besides the Rafale and Mirage 2000 aircraft, Paris will also offer to ensure "AWACS patrols from France around twice a week" if requested, the source said during the minister's visit to Estonian capital Tallinn amid the Ukraine crisis.

U.S. Vice President Joe Biden on Wednesday said Washington could send forces to the Baltic states to reassure ex-Soviet NATO allies rattled by Russia's takeover of Crimea.

"We are exploring a number of additional steps to increase the pace and scope of our military cooperation, including rotating U.S. forces in the Baltic region to conduct ground and naval exercises and training missions," Biden said during a visit to Lithuania, after a first stop in Poland.

U.S. President Barack Obama on Tuesday called for a G7 summit next week to discuss the escalating showdown with Russia over Crimea.

The White House said Obama asked fellow leaders of the grouping -- minus G8 member Russia -- to join him in the Hague, where he will be attending a nuclear security summit, to discuss the crisis and how to support Ukraine.

The United States is sending a dozen F-16 fighter jets to Poland as a part of a training exercise, amid continuing tensions between Ukraine and Russia, the Polish defense ministry said on Sunday.

Three hundred U.S. service personnel will also be sent to Poland as part of the exercise. The deployment will be completed by Thursday.