Too soon to link Boston attack and Chechnya

US Secretary of State John Kerry on Friday refused to speculate on the implications of the reported Chechen roots of two brothers believed to have carried out the Boston bombings.

"I think it's fair to say that for this entire week we've been in pretty direct confrontation with evil," Kerry said after talks with Mexican Foreign Secretary Jose Antonio Meade.

He was speaking as thousands of heavily armed police staged a house-to-house hunt for a teenager suspected of carrying out the Boston marathon bombings, hours after his brother was killed in a shootout.

Dzhokhar Tsarnaev, 19, and his 26-year-old brother Tamerlan were said to be of Chechen origin but were legal residents of the United States.

Kerry, who for almost three decades served as the senator for Massachusetts and has long ties to the Boston Marathon, said people should not jump to conclusions until investigators complete their probe into Monday's attack.

"I'm not going to get into speculation, I'm not going to deal with a hypothetical," the top US diplomat insisted, when asked about the suspects' ties to Chechnya, a mainly Muslim Russian republic in the Northern Caucasus.

"The one thing I will say is: terror is terror. And this underscores the importance of all of us maintaining vigilance and cooperating together internationally.

"Terror anywhere in the world, against any country, is unacceptable. And we need to continue to stand up and fight against it in the way that we are."

He added that the incident "strengthens my resolve" and that the United State was "on the right track."

But Kerry refused to say whether he had already been in contact with Moscow about the brothers' apparent Chechen links.

"At this point, law enforcement officers are carrying out an ongoing investigation and, frankly, they're at some critical stages here," Kerry said.

"It would just be entirely inappropriate for me to be commenting on the tick-tock around the larger issues outside of it."

Russia has sought through two brutal wars to impose calm on the North Caucasus region since 1994, fighting against increasingly Islamist rebels in what Moscow has long insisted is its own "war on terror."