WASHINGTON (REUTERS) - US Defense Secretary Jim Mattis said the killing of former Yemeni President Ali Abdullah Saleh would, in the short term, likely worsen an already dire humanitarian situation in the country.

Saleh was killed in a roadside attack on Monday (Dec 4) after switching sides in Yemen's civil war, abandoning his Iran-aligned Houthi allies in favour of a Saudi-led coalition.

Coupled with a Saudi-led blockade and internal clashes, the stalemate has contributed to a human catastrophe. Some 7 million people are on the brink of famine, while one million are suspected to be infected with cholera.

Mattis, speaking with reporters on a military aircraft en route to Washington after a brief trip to parts of the Middle East and South Asia, said it was too early to say what impact the killing would have on the war.

He said it could either push the conflict towards UN peace negotiations or make it an "even more vicious war". "(But) one thing I think I can say with a lot of concern and probably likelihood is that the situation for the innocent people there, the humanitarian side, is most likely to (get) worse in the short term," Mattis said. He did not explain his reasoning.

The war has already killed more than 10,000 people and displaced millions.

"So this is where we've all got to roll up our sleeves. Now, what are you going to do about medicine and food and clean water and cholera," Mattis said.

"I think there has got to be a lot more focus on the humanitarian side right now."

Analysts said Saleh's death would be a huge moral boost for the Houthis and a serious blow to the Saudi-led coalition that intervened in the conflict to try to restore the internationally recognised government of President Abd-Rabbu Mansour Hadi.

Saudi Arabia and its allies receive logistical and intelligence help from the United States.

Mattis said he did not believe the US military would play a role in easing the humanitarian situation.