Washington (AFP) - US presidential candidates took time from campaigning to reflect on Thursday's deadly mass shooting at an Oregon college, with Democrat Hillary Clinton calling for "sensible" gun law reform.

"It is just beyond my comprehension that we are seeing these mass murders happen again and again and again," Clinton told a pool reporter, adding that lawmakers must forge the "political will" to act on gun legislation.

"I know there is a way to have sensible gun control measures that help prevent violence, prevent guns from getting into the wrong hands, and save lives," the Democratic frontrunner added.

"I am committed to doing everything I can to achieve that."

Republican candidates who spoke out after the shooting offered prayers for the victims and others affected, but they steered clear of addressing gun control, which remains a controversial issue despite many US school shootings each year.

"Praying for Umpqua Community College, the victims, and families impacted by this senseless tragedy," former Florida governor Jeb Bush said.

Retired neurosurgeon Ben Carson, who is polling well in the Republican nominations race, said there would be "those who are going to be calling for gun control."

But that's not the issue, he said on the Hugh Hewitt Show.

"The issue is the mentality of these people" and how to better search for warning signs.

Forcing firearm owners to register every gun would be "very dangerous," he added.

Democratic White House hopeful Martin O'Malley offered a tough message in the Oregon aftermath.

"Tweets won't stop this. Thoughts and prayers won't, either," he posted on Twitter.

"Only real gun reforms will stop mass shootings from occurring nearly every day."