DUBAI/ TEHRAN - Iranian Foreign Minister Mohammad Javad Zarif said on Sunday he could not take up an invitation to visit regional rival Saudi Arabia because the proposed dates clashed with planned nuclear negotiations between Iran and world powers."It is not possible for me to attend," the official IRNA news agency quoted Zarif as saying, explaining that the event to which he had been invited in the kingdom, an Organisation of Islamic Cooperation meeting, coincided with the nuclear talks. His visit would have been the first by a senior Iranian official since President Hassan Rouhani took power in 2013. And, Kuwait's Emir Sheikh Sabah al-Ahmad Al-Sabah on Sunday started a landmark visit to Tehran focused on mending fences between Shiite Iran and the Sunni-ruled monarchies in the Gulf. The two-day visit comes amid a thaw in ties between Tehran and six-nation Gulf Cooperation Council (GCC) since the election of Iran's moderate President Hassan Rouhani in June 2013.In greeting the emir, Rouhani, quoted on the presidency website, said the trip would mark "a decisive turning point" and that the two states had "close views on political, regional and international issues". The visit would "benefit both countries", the emir was quoted as saying.Sheikh Sabah, on his first visit to Tehran as head of state, flew in at the head of a high-level delegation including the foreign, oil, finance, commerce and industry ministers. "Our ties with Kuwait are very important to us and we hope this trip would be a new chapter to boost cooperation," said Iran's Foreign Minister Mohammad Javad Zarif, quoted by state news agency IRNA.The visit will also focus on controversial regional issues, including Iran's military involvement in Syria, the situation in Iraq and Egypt, and the Middle East peace process, Kuwaiti officials said. Relations between Iran and the Gulf, namely signs of rapprochement between regional power brokers Saudi Arabia and Iran, will also be discussed during the visit, Kuwaiti foreign ministry under-secretary Khaled al-Jarallah told Al-Hayat newspaper. He said Kuwait, which currently holds the rotating presidency of the GCC -- which also includes Bahrain, Oman, Qatar, Saudi Arabia and the United Arab Emirates (UAE) -- has balanced links with Tehran and is willing to mediate between Riyadh and Tehran.Saudi Arabia and its GCC partners are deeply suspicious of Iran's nuclear ambitions and wary of the talks under way between Tehran and Western powers aimed at striking a long-term compromise. Riyadh is also at odds with Iran over the Syria war, in which Tehran backs the government and Saudi Arabia the rebels, as well as its involvement in Iraq, Bahrain and other countries in the region.Last month, Saudi Foreign Minister Prince Saud al-Faisal invited his Iranian counterpart Zarif to visit Riyadh. Saudi Arabia has also invited Iran to attend a two-day meeting of the Organisation of Islamic Cooperation that opens on June 18 in Jeddah, with Tehran welcoming the invite as a "friendly" gesture.But Zarif told IRNA on Sunday that he will not be able to attend because the timing coincides with the next round of nuclear talks between Iran and world powers, scheduled for June 16-20 in Vienna. In December, Zarif toured Kuwait, the UAE, Oman and Qatar, but skipped Bahrain and Saudi Arabia. Kuwait's ambassador to Tehran, Majdi al-Dhafiri, told Kuwait's official news agency KUNA that Sheikh Sabah and Rouhani will discuss a number of "strategic projects" useful for the whole region. He did not elaborate.