The fighting in Yemen has expanded from the major cities and ports to a border region with Saudi Arabia. Shelling by Shiite Houthi rebels in the area of Najran in northwestern Yemen has forced Saudi Arabia to suspend school and halt flights into the local airports, according to news reports.

This latest flashpoint comes nearly six weeks into a Saudi-led air campaign to stop the Houthis and their allies, security forces loyal to ousted President Ali Abdullah Saleh, from taking control of Yemen.

The United Nations says hundreds of people have been killed and at least 300,000 displaced since the fighting in Yemen began in mid-March, according to The Associated Press .

The U.N. says the fighting has caused acute shortages of food, water and fuel, according to Reuters , and that it has appealed to Saudi Arabia to stop targeting airports, particularly in the capital, Sanaa, so humanitarian workers can get into the country to deliver aid.

The U.N. humanitarian coordinator for Yemen, Johannes Van Der Klaauw, says coalition strikes have rendered the runways inoperable.

"No flights can take off or land while the runways are being repaired," Van Der Klaauw said. He added that the bombing campaign also has severed humanitarian lifelines at other airports and seaports in Yemen. International aid agencies are hoping to bring in humanitarian aid from nearby Djibouti.

U.S. Secretary of State John Kerry is due to visit the Saudi capital of Riyadh on Wednesday and Thursday for discussions with Saudi officials about a pause in fighting to allow much needed humanitarian assistance to reach Yemen.

Saudi Arabia's new foreign minister, Adel al-Jubeir, said Monday that the coalition was considering a truce in specific areas to allow such deliveries. The official Saudi Press Agency said Jubeir did not give a timeline but warned the Houthi rebels against exploiting a halt in airstrikes, according to Al-Jazeera .

Jubeir also said Saudi Arabia might host a center to coordinate delivery of humanitarian supplies.

Meanwhile, Human Rights Watch says it has credible evidence that the Saudi-led coalition has used cluster munitions supplied by the United States in its airstrikes against the rebels. The organization says cluster munitions pose long-term dangers and are prohibited by a 2008 treaty adopted by 116 countries, but not Saudi Arabia, Yemen or the U.S.

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