"These actions undermine prospects for peace and regional stability and are detrimental to the ongoing diplomatic efforts to end the conflict," Xinhua news agency quoted the UN chief's spokesman Stephane Dujarric as saying on Thursday. 

"We call on all parties to refrain from further fanning the flames of conflict and remain committed to advancing the political process to reach a comprehensive negotiated settlement." 

The UN Office for the Coordination of Humanitarian Affairs (OCHA) said that significant displacement has been reported in the conflict-hit Marib Governorate, especially in Sirwah district, as fighting has continued along several frontlines. 

More than 14,000 people have been displaced. 

Aid agencies warn that as many as 385,000 men, women and children could be forced to flee due to the offensive, OCHA said. 

Many of them may have to go to crowded displacement sites where services are overly stretched. 

"We, along with our partners, continue to respond and have reached more than 11,000 people with food baskets, emergency shelter kits and other forms of urgent life-saving assistance," said Dujarric. 

He added that there is more concern over a wide increase in action by all parties to the Yemen conflict, including airstrikes. 

"We are deeply concerned by the recent reports of increased Houthi cross-border attacks against Saudi Arabia,. 

"We are aware of reports of an alleged Houthi (who also call themselves Ansar Allah) attack earlier today (Thursday) against a Saudi Aramco facility in Jeddah." 

While unable to comment on the veracity of the claims, the spokesman said: "We reiterate our condemnation of all attacks targeting civilians and civilian infrastructure, which are in violation of international humanitarian law." 

Dujarric said discussions continue on several pending logistical issues for the mission to inspect and carry out emergency repairs for the derelict oil tanker, the Safer. 

It is loaded with 1.1 million barrels of oil and is moored just offshore. 

There is fear the rusting hulk could leak, causing an environmental disaster. 

The UN recently received permits for mission personnel to travel to Yemen, he said. 

"But so far, we don't have concrete solutions for some of the other pending issues. Until those other issues are resolved, we are not in a position to spend more donor money to rent a vessel or estimate a new timeline for the mission." 

Yemen has been mired in civil war since late 2014 when the Iran-backed Houthi rebels seized control of several northern provinces and forced the Saudi-backed government of President Abd-Rabbu Mansour Hadi out of the capital Sanaa. 

Due to the ongoing war, the country currently faces the largest humanitarian crisis in the world, with more than 24 million people, some 80 per cent of the population, in need of humanitarian assistance, including more than 12 million children.