BRUSSELS, March 25 (Reuters) - Belgian police arrested seven people and Germany arrested two in investigations into Islamic State suicide bombings in Brussels, while authorities in France said they thwarted a militant plot there "that was at an advanced stage."
Islamic State suicide bombers hit Brussels airport and a metro train on Tuesday, killing at least 31 people and wounding some 270 in the worst such attack in Belgian history.
Investigators believe the attacks were carried out by the same Islamic State cell responsible for gun and bomb attacks that killed 130 people in Paris in November.
The Belgian federal prosecutor's office said six people were held during searches in the Brussels neighborhoods of Schaerbeek in the north and Jette in the west, as well as in the center of the Belgian capital. Public broadcaster RTBF said a seventh man was arrested in the Forest borough of Brussels early on Friday.
Germany's Der Spiegel magazine said German police had arrested two people. One had received phone messages with the name of the metro station bomber and the word "fin" -- French for "end" -- three minutes before the metro blast, it said. The German interior ministry declined immediate comment.
The Belgian daily De Standaard said on Friday police had arrested a man who was filmed by security cameras in the airport terminal next to two bombers who blew themselves up there. Prosecutors did not confirm the arrest and it was not known if the man was among the seven detained overnight.
The attack in Brussels, home to the European Union and NATO, has heightened security concerns around the world and raised questions about EU states' ability to respond in an effective, coordinated way to the Islamist militant threat.
U.S Secretary of State John Kerry arrived in Brussels on Friday to offer U.S. assistance in security. U.S. officials said two of the Brussels victims were Americans. China and Britain said one each of their nationals were among the dead.
"The United States is praying and grieving with you for the loved ones of those cruelly taken from us, including Americans, and for the many who were injured in these despicable attacks," Kerry said after meeting Prime Minister Charles Michel.
"Je suis bruxellois. Ik ben Brussel," Kerry said after brief remarks in French and Dutch, expressing solidarity in its two languages that he too felt a citizen of the Belgian capital.
In Paris on Thursday, authorities arrested a French national suspected of belonging to a militant network planning an attack in France, although they said there was no evidence directly tying his plot to the Brussels and Paris attacks.
Interior Minister Bernard Cazeneuve said in a televised address that the arrest helped "foil a plot in France that was at an advanced stage."
A French Interior Ministry wanted notice published by French media named him as Reda Kriket, 34, who was sentenced in absentia to 10 years in prison in Belgium last July for recruiting Islamist fighters for Syria.
After the arrest by the French counterterrorism service, DGSI, the agency raided an apartment building in the northern Paris suburb of Argenteuil. A police source said investigators found acetone peroxide explosives in the apartment.
Belgium's interior and justice ministers offered to resign on Thursday over a failure to track one of the airport bombers, Brahim El Bakraoui, 29, who had been expelled last year by Turkey as a suspected fighter. Bakraoui's brother Khalid, 26, was the bomber who struck Maelbeek metro station.
Interior Minister Jan Jambon and Justice Minister Koen Geens tendered their resignations to Prime Minister Charles Michel, who asked them to stay on. "In time of war, you cannot leave the field," said Jambon, a right-wing Flemish nationalist.
Turkish President Tayyip Erdogan said Brahim El Bakraoui had been expelled in July after being arrested near the Syrian border, and two officials said he had been deported a second time. Belgian and Dutch police had been notified of Turkish suspicions that he was a foreign fighter trying to reach Syria.
At the time, Belgian authorities replied that Bakraoui, who had skipped parole after serving less than half of a nine-year sentence for armed robbery, was a criminal but not a militant.
"You can ask how it came about that someone was let out so early and that we missed the chance to seize him when he was in Turkey. I understand the questions," Jambon said. "In the circumstances, it was right to take political responsibility and I offered my resignation to the prime minister."
The Brussels attacks came a week after Belgian police killed a militant during a house raid that led them to Salah Abdeslam, the only suspected participant in the Paris attacks to have been captured alive.
Belgian public broadcaster VRT said investigators believed that Abdeslam had probably planned to carry out a shooting and suicide bomb attack in Brussels, similar to Paris.
The news website Politico Europe said investigators had only questioned Abdeslam for one hour in the four days between his arrest and the Brussels bombings.
Belgian daily De Morgen said investigators had identified a new suspect they believe played a role in the Brussels bombings, 28-year-old Syrian Naim al-Hamed. The paper said he was on a list circulated to the security services of other European countries after Tuesday's attacks, and was also suspected of involvement in the Paris attacks.
Security sources told Belgian media one of the suicide bombers at the airport was Najim Laachraoui, a veteran Belgian Islamist fighter in Syria suspected of making explosive belts for November's Paris attacks.
A police wanted notice for Laachraoui was removed from the Belgian police website on Friday, reinforcing the view that he did blow himself up in Tuesday's attacks.
Belgium on Thursday lowered its security alert level one notch to three from the highest level, four, but officials did not say what that would mean in terms of security measures that have included a heavy police and military presence in Brussels.
Islamic State posted a video on social media calling the Brussels blasts a victory and featuring the training of Belgian militants suspected in the Paris attacks.
(Additional reporting by Miriam Rivet, Geert De Clercq, Matthias Blamont and John Irish in Paris, Philip Blenkinsop, Julia Fioretti, Barbara Lewis, Bate Felix, Jan Strupczewski, Robin Emmott and Jean-Baptiste Vey in Brussels; writing by Peter Cooney, Paul Taylor and Peter Graff; Editing by Mark Heinrich and Hugh Lawson)