"We will continue to provide any assistance necessary in investigating these heinous acts of terrorism and bringing those responsible to justice ... We offered today anything they need and they are already taking us up on (it)," Kerry said.
"We will specifically work with them to help in the coordination of the flow of information."
He added that EU governments had been working over the past year to better exchange counter-terrorism information. "There is room for additional coordination," he said.
Belgium has drawn international criticism for security lapses in the run-up to the bombings in Brussels airport and a metro station which, in addition to the 31 dead, injured around 270 people. The U.S. official declined to name the Americans killed until their next of kin had been notified.
They were later identified as Alexander and Sascha Pinczowski, a brother and sister who were Dutch citizens and lived in New York.
Kerry took issue with the criticism of Belgium. "This carping four days later is a little frantic and inappropriate. People are jumping to conclusions, they are looking for things that sometimes just are difficult to analyze in the immediate aftermath."
He told reporters he did not know whether opportunities had been missed by Belgian authorities in tracking Islamic State militants, but added: "This government has been in place for a year and it has moved very forcefully to deal with terrorism."
Kerry said the United States was working with the Belgian government before the attacks and had a "foreign fighter surge team" in Brussels in February to work on specific counter-terrorism efforts.
There were also a number of counter-terrorism initiatives already scheduled to take place in Brussels before the attacks occurred to help detect suspected jihadists, he said.
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