CAIRO (AP) — The Latest on developments in Yemen (all times local):

5 p.m.

Saudi Arabia's powerful crown prince says a ballistic missile launched at the kingdom by Shiite rebels in Yemen last weekend was a "direct military aggression by the Iranian regime and may be considered an act of war."

The state-run Saudi Press Agency carried the remarks Tuesday in an article about a call between Crown Prince Mohammed bin Salman and British Foreign Minister Boris Johnson.

Saudi Arabia has accused Iran of supplying the ballistic missile that the Houthi rebels and their allies fired toward Riyadh's international airport on Saturday night. Iran, which supports the Houthis but denies arming them, says it had nothing to do with the attack.

A Saudi-led coalition has been fighting the rebels since March 2015 in support of Yemen's internationally recognized government.

Saudi Arabia has intensified its blockade on Yemen over the launch, cutting off all traffic to Yemen's air and sea ports and closing land crossings

1:45 p.m.

A United Nations official says all of its humanitarian flights to Yemen have been grounded after flights were no longer given clearance from the Saudi-led coalition to land in the country.

Doctors Without Borders said on Tuesday its flight from Dibjouti to the Yemeni capital Sanaa has also been canceled for the same reason.

The coalition ordered all ports — air, land and sea — closed after Shiite rebel Houthis fired a ballistic missile at the Riyadh airport. The Saudis have accused Iran of arming the Houthis and considered the assault an "act of war."

The official said on Tuesday that the U.N. is "in touch at the highest level to resolve the issue as soon as possible." She spoke on condition of anonymity because she wasn't authorized to speak to the press.

Yemen has been already under a blockade since March 2015, when the coalition waged a war on Houthis. The latest measures have tightened the blockade which contributed to Yemen's crisis, the world's worst humanitarian crisis.

— By Maggie Michael

12:30 p.m.

A Houthi-linked army spokesman in the Yemeni capital has threatened escalation against Saudi Arabia and the United Arab Emirates, saying the rebels consider the two countries' airports "legitimate targets."

Col. Aziz Rashed told reporters late Monday in Sanaa that his military experts are able to develop missiles with ranges that exceed 1500 kilometers.

The threat comes as part of the mutual escalation between Yemen's Houthi rebels, in control of the northern part of the country, and the oil-rich gulf neighbors who went to war against them after rebels forced internationally-recognized government to flee the country in 2015.

Rashed says: "We urge all airline companies and travelers to avoid airports in both Saudi and UAE as we consider them a legitimate military targets within the range of our ballistic missiles."