US Secretary of State John Kerry stopped in Brussels Friday morning in a show of solidarity with European allies in the fight against terrorism.

"Je suis Bruxellois" -- "I am a citizen of Brussels," Kerry said, recalling that after the 9/11 terror attacks in the US "voices across Europe declared, 'Je suis Americain.'"

Tuesday's bombing attacks at Brussels' airport and a metro station killed 31 people and injured 300. It now appears that at least two Americans are among the dead. The "Islamic State" claimed responsibility for the attack.

Kerry met with Belgian and European Union officials, including Prime Minister Charles Michel and European Commission President Jean-Claude Juncker in the morning.

Michel thanked Kerry for his visit, calling it a powerful message of solidarity. "It is very important for us today to receive your support," he said.

The prime minister expressed condolences to Kerry for the American victims and vowed to step up counterterrorism cooperation with the US and other countries.

"We want to cooperate with you," Michel said, "do our best with you in order to face these very sensitive issues."

Resignations rejected

Belgian authorities have come under fire for failing to prevent the attacks amid a series of missed signals. The interior and justice ministers offered their respective resignations, but they were rejected by Michel.

Hours before Kerry's arrival, authorities arrested six suspects in connection with this week's bombings. Meanwhile, the manhunt continues for a lone suspect in Tuesday's attack.

Kerry, who was returning to Washington after meetings in Moscow, expressed his condolences to his Belgian ally.

"The United States stands firmly with Belgium and with the nations of Europe in the face of this tragedy," Kerry said after meeting with Michel.

"We will continue to provide any assistance necessary in investigating these heinous acts of terrorism and bringing those responsible to justice," Kerry added.

The US' top diplomat said that the forces aligned against IS extended beyond the US-European alliance and cited Turkey and Tunisia – two predominantly Muslim countries that have also been targeted by the IS – as just two of many allies in the battle against terrorism.

"We - all of us representing countless nationalities - have a message for those who inspired or carried out the attacks here or in Paris, or Ankara, or Tunis, or San Bernardino, or elsewhere: We will not be intimidated," he said. "We will not be deterred. We will come back with greater resolve - with greater strength - and we will not rest until we have eliminated your nihilistic beliefs and cowardice from the face of the Earth."

bik/jil (AP, AFP, Reuters, dpa)