The White House warned Monday that attacks on the Boston marathon should not derail momentum towards immigration reform, President Barack Obama's best shot at a big second-term legacy achievement.

The fact that the two suspected perpetrators of the bombings were of Chechen origin and were living legally in the United States has led some reform advocates to fear their actions could be used to slow the drive for change.

Asked whether the aftermath of the Boston attacks could slow momentum behind a Senate effort to pass comprehensive new immigration laws, White House spokesman Jay Carney said: "I would simply say that it should not."

"One of the positive effects and one of the reasons why we need comprehensive immigration reform is because it will enhance, when implemented, our national security," said Carney.

"One of the reasons why we need comprehensive immigration reform is we need to bring out of the shadows the roughly 11 million residents of this country who are here illegally.

"The process of moving along the earned path to citizenship and the various hurdles that have to be cleared in that process, allows for much more information to be known by the relevant authorities and agencies about these individuals."

"It also enhances the entry and exit procedures that are part of the immigration process."

Suspects Dzhokhar Tsarnaev, 19, and his brother Tamerlan, 26, have lived in the United States for over a decade.

The older brother, killed after a shootout with police last week, was a US resident who had applied, but not yet been granted US citizenship, reports said.

His younger brother, who is under armed guard in a Boston hospital, became a US citizen on September 11, 2012.

A bipartisan group of senators last week unveiled the latest attempt to reform creaking US immigration laws.

Immigration reform is currently being examined in committee ahead of a crucial debate due to begin within weeks, and faces a tough test from opponents who warn granting legal status to undocumented immigrants equates to amnesty.