Prime Minister Shinzo Abe visited Washington's Holocaust Memorial on Monday to hail a rare hero of Japan's brutal World War II past.

Previously, Abe has faced criticism for his allegedly revisionist views of Japan's own war-time behavior.

Ten pro-Kremlin bikers on a controversial ride to Berlin to celebrate Soviet victory in World War II were on Monday denied entry into Poland at the Belarusian border, Polish border guards said. 

The bikers include members of the Night Wolves, a fiercely nationalist club closely linked to Russian President Vladimir Putin.

Poland on Friday said it had denied entry to a Russian biker gang backed by the Kremlin that plans to ride through Europe to celebrate the Soviet Union's role in the World War II victory over Nazi Germany. 

The planned two-week, 6,000-kilometer (3,728-mile) rally by Russian bikers including the Night Wolves -- a fiercely nationalistic motorcycle club backed by Russian President Vladimir Putin -- had sparked anger in Poland.

The United States welcomed Tuesday news that Poland is planning to buy the U.S.-made Patriot missile system in an unprecedented revamp of its military in a deal worth up to $5 billion.

"Poland is a stalwart NATO ally, and its defense modernization program... directly fortifies the military strength of the alliance," State Department acting spokeswoman Marie Harf said in a statement.

Poland said Tuesday it will hold exclusive talks with the U.S. government on a multi-billion-euro Patriot missile contract and test Airbus helicopters in an unprecedented military upgrade amid heightened tensions with neighboring Russia.

President Bronislaw Komorowski said the decision for the massive defense revamp is being made at a time of "deterioration of security due to the Russian-Ukraine conflict."

Former SS officer Oskar Groening, dubbed the "bookkeeper of Auschwitz", asked for "forgiveness" over his role in mass murder at the Nazi death camp, as his German trial began Tuesday.

"For me there's no question that I share moral guilt," the 93-year-old former Nazi told the judges, admitting that he knew about the gassing of Jews and other prisoners.

The U.S. ambassador to Warsaw apologized Sunday after being summoned by the foreign ministry over comments by the head of the FBI that Poland shared responsibility for the Holocaust with Germany.

Poland's President Bronislaw Komorowski said the comments in an opinion piece by FBI director James Comey were an "insult" to Poland.

Poland's prime minister on Wednesday criticized a fiercely nationalistic Russian biker gang's plans to ride through Europe to celebrate the Soviet Union's victory over Nazi Germany in World War II. 

The two-week 6,000-kilometer (3,750-mile) rally by Russian bikers including the Night Wolves, a motorcycle club backed by Russian President Vladimir Putin, comes as tensions run high between Russia and the West over the crisis in Ukraine.

Poles on Friday marked the fifth anniversary of the deadly plane crash in Russia that killed then president Lech Kaczynski and 95 officials, amid continued controversy over the causes.

With the May 10 presidential elections just around the corner and fresh leaks of cockpit transcripts, opposition politicians allege the accident was a political assassination that the government is trying to hush up.

Ukrainian soldiers fighting pro-Russian separatists in east Ukraine are also fighting to ensure the security of Europe, Poland's President Bronislaw Komorowski said in a speech to the Kiev parliament Thursday.

"Today when the sons of Ukraine are killed in eastern Ukraine protecting their independence, they also protect Europe. They protect it from the return of imperial thinking, from decisions that threaten the whole of Europe," he said.