The United States is concerned about the worsening humanitarian situation in Yemen and will discuss a possible pause in fighting with Saudi officials on Wednesday to try to get food, fuel and medicine to civilians, Secretary of State John Kerry said.

"The situation is getting more dire by the day and we are concerned about that," Kerry told a news conference during a visit to Djibouti, the first secretary of state to visit the Horn of Africa nation.

Kerry said the United States had urged both sides in the Yemen conflict to comply with humanitarian laws and to ensure that civilians were spared in the fighting.

He announced $68 million in new aid for relief agencies working in Yemen as humanitarian groups warned fuel shortages could affect their efforts to tackle the crisis.

A shortage of fuel has crippled hospitals and food supplies in the past weeks, and the U.N.'s World Food Programme has said its monthly fuel needs have leapt from 40,000 litres a month to 1 million litres.

A Saudi-led Arab coalition began air strikes in Yemen on March 26 against Iran-allied Houthi fighters, backed by forces loyal to former President Ali Abdullah Saleh, who seized control of parts of the country, including the capital Sanaa.

The Saudi-led alliance has said it is considering a truce in some areas to allow for humanitarian supplies.

"We will be discussing the nature of the pause and how it might be implemented. I am convinced of their desire to implement the pause," Kerry said.

He said he had discussed the idea with other countries, which suggested Houthi fighters "might also be willing to engage in a pause."

"This will be welcome news for the world if it were able to be affected, if it doesn't see people try to take advantage of it," he said.

Kerry met Djibouti's President Ismail Omer Guelleh and discussed his country's role as a center for Washington's interventions against extremism in Africa and Islamist militant safe-havens in Yemen. "We could not be doing what we're doing today without the help from Djibouti," he said.

He also visited the U.S. military base from where pilots fly missions over Yemen and Somalia.

The United States has doubled the number of consular staff in Djibouti to help Yemeni-American families fleeing the conflict in Yemen, and as of April 28 had assisted more than 500 U.S. citizens, the State Department said.

The United Nations said on Tuesday the conflict in Yemen had killed at least 646 civilian since Saudi-coalition air strikes began, including 131 children. More than 1,364 civilians had been wounded.

Saudi Arabia says the campaign is aimed at restoring President Abd-Rabbu Mansour Hadi's government. The Houthis say their campaign was aimed at fighting al Qaeda militants and to combat corruption.