A proposal to expand federal funding for stem-cell research using human embryos was rejected by US President George W Bush on Wednesday.

"This is certainly not an attempt to muzzle science," White House spokesman Tony Snow said Wednesday. "But it is an attempt, I think, to respect people's conscience on such an issue."

This is Bush’s third veto in six-and-a-half years as president. Last July, he rejected a similar bill passed by a Republican-controlled Congress.

The stem cell legislation is considered a top priority by the Democrats from when they took control of the House and Senate in January,

"The president supports and encourages stem cell research, including using embryonic lines, as long as it does not involve creating, harming or destroying embryos," spokesman Tony Fratto said. "That is an ethical line that should not be crossed."

Instead, Bush signed an executive order Wednesday encouraging federal research institutions to fund more stem-cell research that uses sources other than human embryos, the White House added.

Democrats condemned Bush's veto. Democratic Congressman Rahm Emanuel called it "a crushing blow for millions of Americans" hoping for medical advances to treat their illnesses.

"This is just one example of how the president puts ideology before science, politics before the needs of our families, just one more example of how out of touch with reality he and his party have become," Sen. Hillary Rodham Clinton, D-N.Y., told the Take Back America conference of liberal activists Wednesday.

In order to overcome Bush’s veto democrats would need two-thirds majority votes in the Senate and House of Representatives. 

According to its backers, stem cell research shows promise for the treatment of degenerative diseases such as Alzheimer's and Parkinson's and for diabetes.