* Unclear how the Houthis will impose Saudi blockade

* War risk rates creeping higher already - sources

* Somali piracy in Gulf of Aden adding to dangers for ships

By Jonathan Saul

LONDON, July 20 (Reuters) - The insurance cost of shipping

goods through the Red Sea rose on Monday after Yemen's

Iran-aligned Houthis said they were imposing a naval blockade on

Saudi Arabia, escalating risks for merchant shipping, insurance

industry sources said.

The move by the Houthis opens a new front against the United

States in its war on Iran and widens the threat to global energy

supplies and trade beyond the Gulf.

Indicative war risk premiums rose to around 0.75% of the

value of a ship, from around 0.3% on Friday before the Houthi

announcement, according to the sources, who declined to be

identified due to the sensitivity of the matter.

Even a small change will mean hundreds of thousands of

dollars in extra costs for a seven-day voyage.

It is not clear how the Houthis would carry out a maritime

blockade of Saudi Arabia, its northern neighbour along the Red

Sea coast, or whether it would include a return to attacks on

shipping.

Saudi state oil giant Saudi Aramco, the world's

largest oil exporter, has increased use of the Yanbu terminal in

the Red Sea since the U.S.-Israeli conflict with Iran began on

February 28.

Saudi Arabia-flagged, owned or operated vessels, vessels en

route to or from Saudi Arabia and Saudi Arabia's Red Sea ports

were assessed at high risk of a Houthi attack, British maritime

security company Ambrey said on Monday.

"The Houthis made mistakes during the (2024) Red Sea crisis

in targeting shipping with out-of-date affiliations to

companies. It is likely that vessels could be targeted for

mistaken identities," Ambrey said.

The full closure of the Bab el-Mandeb strait, the southern

gateway linking the Red Sea to the Gulf of Aden, would halt

Saudi oil exports to Asia and could reduce global oil supply by

7%.

Red Sea traffic has not fully recovered since Houthi attacks

off Yemen's coast began in November 2023 in what the group said

was solidarity with Palestinians in the Gaza war.

Some Houthi attacks continued until mid 2025, only ending

completely with the Gaza ceasefire in October last year.

Suspected Somali piracy and ship hijackings have also risen

in recent months in the Gulf of Aden, adding to further dangers

for shipping.