Protesters carry banners as they take part in a protest against Saudi-led air strikes on Yemen, in front of the offices of the UN headquarters in Beirut May 12, 2015. [Photo/Agencies]

CAIRO/ADEN - Saudi-led air strikes hit the rebel-held Yemeni capital Sanaa hours before a five-day humanitarian truce took effect on Tuesday, and Washington cautioned against "provocative actions" after Iran dispatched a cargo ship to Yemen.

The ceasefire began at 11 p.m. (2000 GMT), said Brigadier General Ahmed Asseri, spokesman for a Saudi-led coalition which has been striking the country's Houthi rebels since March 26.

It is intended to allow the shipment of food and medicine to the country, which aid groups warn faces a humanitarian catastrophe after more than seven weeks of war.

There was no let up in fighting, despite the imminent truce.

The Houthis shelled Saudi border areas in Jizan province until the last moments before the ceasefire started, Asseri said on al-Arabiya television, adding that this gave him no confidence the rebel group intended to keep to the truce.

And as the ceasefire neared, witnesses said the Saudi-led alliance bombed Houthi positions in the southern port of Aden, where local armed groups were still fighting the rebels.

The United States said it was tracking Iranian warships accompanying the vessel bound for Hodaida port, and urged Iran instead to use a UN distribution hub in Djibouti to provide help to people in the war-damaged Arabian Peninsula country.

"We would discourage any provocative actions," said US State Department spokesman Jeff Rathke.