July 21 (Reuters) - Major Gulf stock markets were mixed early on Tuesday, as investors weighed reports of U.S.-Iran mediation against fresh attacks and Houthi threats to impose a naval blockade on Saudi Arabia.

Dubai's main share index .DFMGI added 0.3%, with blue-chip developer Emaar Properties EMAR.DU gaining 1.3% and toll operator Salik SALIK.DU rising 1.6%.

In Abu Dhabi, the index .FTFADGI rose 0.5%.

Saudi Arabia's benchmark index .TASI fell 0.3%, hit by a 1.6% slide in Al Rajhi Bank 1120.SE and a 0.5% decrease in oil major Saudi Aramco 2222.SE.

Brent crude futures LCOc1 fell 1.1% to $88.26 per barrel by 0630 GMT. O/R

Yemen's Iran-aligned Houthis threatened a naval blockade of Saudi Arabia, risking a wider conflict and greater disruption to energy supplies and trade. The move came even as Tehran and Washington signalled interest in renewed diplomacy, with Iran saying mediators had presented new proposals but giving no details.

Saudi Arabia on Monday condemned what it described as allegations by Yemen's Iran-aligned Houthis that the kingdom was besieging the Yemeni people, after the group announced it would impose a maritime blockade on Saudi Arabia.

The Qatari index .QSI eased 0.1%, with telecom operator Ooredoo ORDS.QA losing 1.5%.