War-torn Yemen is in imminent danger of the worst famine the world has seen in decades, UN Secretary General Antonio Guterres warned Friday.

"In the absence of immediate action, millions of lives may be lost," Guterres said of the country that has endured a five year war between Iranian-backed Huthi rebels and government forces.

The government in Yemen is supported by a Saudi-led coalition, assisted by Western powers including the US.

The warning, the latest in a series from the UN on Yemen, comes amid reports that President Donald Trump is considering labeling the Huthis a terrorist organization.

That could cripple aid deliveries and make things even worse in Yemen, aid groups say.

In his statement, Guterres made only indirect reference to the possibility.

"I urge all those with influence to act urgently on these issues to stave off catastrophe, and I also request that everyone avoids taking any action that could make the already dire situation even worse," the secretary-general said.  

He said the reasons for the heightened threat of famine include a sharp drop in funding for the UN-coordinated relief program, the instability of the Yemeni currency and the warring parties imposing "impediments" for relief organizations.