Iranian President Hassan Rouhani has told the Emir of Qatar that a "siege" imposed on it by four Arab states that recently cut ties with Doha "is not acceptable for us", according to the office of the president's website.
A statement released on Sunday quoted Rouhani as telling Emir Sheikh Tamim Bin Hamad Al Thani in a phone conversation that "Tehran will stand by Qatar's government".
"Iran's air space, ground and sea will be always be open to Qatar as a ... friendly nation," said Rouhani, adding that the cooperation of the two countries will remain "continuous".
"We believe that if there are disagreements among countries of the region, pressure, intimidation, and sanction are not good ways for settle the disagreements," the statement added.
Sheikh Tamim Bin Hamad Al Thani extended wishes for the Eid al-Fitr Muslim festival, which began on Sunday, and said: "Qatar has open arms for interaction and cooperation."
He added: "Relations between Iran and Qatar have always been developing and powerful."
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Iran's regional rival Saudi Arabia, alongside the United Arab Emirates, Bahrain and Egypt, cut diplomatic and trade ties with Qatar and imposed a land, air and sea blockade against it on June 5, accusing it of supporting "terrorism".
The four countries have not provided any evidence, and Qatar has repeatedly denied the allegations as baseless.
Qatari officials immediately dismissed the document, which also demanded Doha shut down the Al Jazeera Media Network and close a Turkish military base in Qatar, as neither reasonable nor actionable.
Rouhani's statement came hours after Turkish President Recep Tayyip Erdogan denounced the demands of the Saudi-Emirati-led bloc as being "against international law".
Also on Sunday, US Secretary of State Rex Tillerson said some of the demands "will be very difficult for Qatar to meet".
Tillerson added, however, that the list contained "significant areas which provide a basis for ongoing dialogue leading to a resolution".
The top US diplomat also said Washington was backing mediation efforts by Kuwait aimed at defusing the crisis and urged the different sides to "sit together" and continue discussing.