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The Saudi-led coalition against Houthi rebels in Yemen warned a five-day truce "would not last long" in the face of alleged breaches, as the UN called for calm to allow badly needed aid to reach the country.

The coalition accused the rebels of violating the ceasefire for the second day in a row by carrying out military operations, including shelling Saudi troops in the border zone and targeting citizens' homes, according to a statement carried by the official Saudi Press Agency.

The coalition "warns the Houthi militias and their supporters that self-restraint and being committed to the truce would not last long in (the) case the militias continue such practices," it said.

The humanitarian pause that began late Tuesday is the first break in the air war the coalition began on March 26 in support of exiled President Abedrabbo Mansour Hadi.

The Saudi-led campaign was launched to reinstate the rule of Hadi after the Houthis, backed by army units loyal to ex-president Ali Abdullah Saleh, overran much of Yemen.

The bombings have taken a heavy toll, with the United Nations estimating that more than 1,500 people have been killed in airstrikes and fighting between rebel forces and Hadi loyalists since March.

UN envoy to Yemen Ismail Ould Sheikh Ahmed said he was "very worried about violations of the truce", urging all sides to "strictly respect a cessation of military operations... to allow the flow of aid" and to spare airports, ports and any other infrastructure necessary to allow help to reach the embattled population.

Residents said calm prevailed across most of the country except in the cities of Taez, Daleh and oil-rich Marib, where they reported intermittent exchanges of fire between rebel and pro-Hadi forces.