BRUSSELS (AFP) - US Secretary of State John Kerry on Friday (March 25) declared "Je suis Bruxellois" - "I am a citizen of Brussels" - in support for the people of the Belgian capital, echoing their backing for the United States after the Sept 11 terror attacks.

"Then, voices across Europe declared, 'Je suis Americain'. Now, we declare, 'Je suis Bruxellois' and 'Ik ben Brussel'," Mr Kerry said in French and Dutch, the country's two main languages, after meeting Belgian Premier Charles Michel.

Mr Kerry said a number of Americans were among the dead in Tuesday's bomb attacks on Brussels airport and the metro system which were claimed by the Islamic State in Iraq and Syria (ISIS) group.

Neither Mr Kerry nor Mr Michel gave a figure for those killed but a US official separately said two US citizens had been confirmed dead.

The triple blasts left 31 people dead and 300 injured.

"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," Mr Kerry said.

Mr Kerry arrived in Brussels from Moscow where he held talks with Russian President Vladimir Putin on bolstering a fragile ceasefire in Syria where ISIS has seized swathes of territory and where many of its European extremists have been been hardened on the battlefield.

The secretary of state said the victims of Tuesday's attacks were "not combatants in any conflict; they posed no threat; they wished no one harm," in stark contrast to "terrorists whose sole aim was to kill and maim and sow fear".