WASHINGTON (AFP) - The US Supreme Court ruled on Monday it was illegal to buy a gun on behalf of someone else who did not go through the background check.

The ruling comes on the heels of a spate of US shootings that have again sparked debate over gun control, with President Barack Obama calling for national "soul searching" over gun violence.

The top court ruled five to four against a former Virginia policeman, Bruce Abramski, who made a so-called "straw purchase" of a handgun for his uncle, who was in Pennsylvania.

Although both men were legal gun owners, Abramski had indicated on a federal form that he was the gun's actual buyer.

During the Jan 22 oral arguments, Abramski's lawyer, Richard Dietz, contended the transaction was legal because the men's legal and mental background allowed either of them to purchase a firearm.

But justice Elena Kagan responded at the time: "It doesn't matter if the ultimate transferee is Al Capone or somebody else."

"The purpose is to take away guns from (persons with) mental illnesses."

Judge Kagan responded again on Monday in giving the court's majority decision, summarizing the opinion of the court's four progressive justices alongside more conservative justice Anthony Kennedy, who has often been a swing vote.

"No piece of information is more important under federal firearms law than the identity of a gun's purchaser," Judge Kagan emphasised, saying it was "fundamental to the lawfulness of a gun sale" to respond truthfully on the federal form.

But in their dissent, the four more conservative judges said it was enough that the buyer and the eventual owner were both legally eligible to own guns.

This is the second Supreme Court ruling this year on gun control issues. In March, the court upheld a federal law barring anyone convicted of even a minor domestic violence charge from ever owning a gun.