DOHA, May 1 (Reuters) -

Qatar, which hosts the largest U.S. air base in the Middle East, has spoken out against Washington's decision to block all exports of Iranian oil, saying unilateral sanctions were unwise because they hurt the countries that rely on the supplies.

The United States has demanded that buyers of Iranian oil stop purchases by May 1 or face the prospect of sanctions, ending six months of waivers that had allowed Iran’s eight biggest customers, most in Asia, to import limited volumes.

"The sanctions should not be extended because they have an adverse impact on countries benefiting from Iranian oil," Qatar's foreign minister Sheikh Mohammed bin Abdulrahman al-Thani said on Wednesday.

"In Qatar, we do not believe unilateral sanctions bring positive effects for crises which must be solved through dialogue and dialogue only," he told a news conference following a meeting of the Asia Cooperation Dialogue in Qatar's capital Doha, which was attended by his Iranian counterpart.

Qatar, the world's leading exporter of liquefied natural gas, is at odds with other Gulf Arab states who are strong supporters of tighter U.S. sanctions on Iran.

Saudi Arabia and its allies accuse Qatar of supporting terrorism, which Qatar denies, and of cosying up to their regional foe Iran. They cut trade and diplomatic ties with Qatar in 2017, a boycott that Qatar says is aimed at curtailing its sovereignty.

The White House has said it is working with Saudi Arabia and the United Arab Emirates to ensure oil markets, which have already tightened this year due to supply cuts led by the Organization of the Petroleum Exporting Countries (OPEC), were “adequately supplied".

Qatar, which despite its large gas exports sells comparatively little oil, quit OPEC in December, a move seen as a swipe at the organization's de facto leader Saudi Arabia.

Officials from Saudi Arabia and Bahrain attended Wednesday's conference in Qatar, the first time they have done so since the boycott began. Sheikh Mohammed called their participation "limited" and said there was no sign of a thaw in relations.

"Unfortunately, we still see the same behavior of the blockading states of stubbornness and denial. We hope that one day they will go back to wisdom and will come back to the table and address the grievances in front of us," he said.

In November, the minister said Doha would continue to deal with Iran, which helped Qatar secure supplies when the boycott was first imposed, and that it was ready to mediate between Washington and Tehran.