Fierce battles have been raging between government troops and the Houthi rebels from the Zaidi sect of Shia Islam in the north-western provinces of Saada and Amran since the military launched a major offensive on August 11.
Aid groups, who have been given limited access to the northern provinces, say up to 150,000 people have fled their homes.]
Besides the unrest in the north, separatists in southern Yemen are fighting for autonomy and al-Qaeda-linked groups have stepped up attacks across the country.
Meanwhile, thousands of supporters of the opposition Southern Movement held a rally in the town of Radfan on Wednesday, witnesses said.
The opposition website Aden Press said Ali Salem al-Beidh, the exiled former president of South Yemen, addressed the rally by telephone from Germany.
"We will not retreat from the aim of realising our second independence, whatever the sacrifices are and however long it takes," al-Beidh was quoted as saying.
"The British occupation came from overseas, while the current one was the result of a coup by the Sanaa regime," he added, referring to a brief civil war in 1994.