* Riyadh accused Iran of encroaching on its oil, gas fields
* Tensions over Gulf boundaries, Saudi Shi'ite unrest, Syria
DUBAI, Nov 20 (Reuters) - Iran accused Saudi Arabia on Tuesday of conducting exploration activities in prohibited border areas, hitting back at its major rival for regional influence after Riyadh complained of Iranian encroachments on its oil and gas fields in the Gulf.
Separated by 250 km (150 miles) of Gulf waters, Shi'ite Muslim power Iran and Sunni Muslim-led Saudi Arabia have tense relations. Riyadh accuses Iran of fomenting unrest among Shi'ites in its oil-rich Eastern Province, a charge Iran denies, and the two support opposite sides in Syria's civil war.
"Apparently Saudi Arabia has taken action for exploration activities in prohibited border areas," Iranian Foreign Ministry spokesman Ramin Mehmanparast said at his weekly news conference in Tehran, according to the official news agency IRNA.
"The necessary notices were given, and our country's point of view and our commitment to border agreements were conveyed to the Saudi ambassador in Tehran."
Mehmanparast was responding to a question about a Saudi complaint to the United Nations last week saying that Iran strayed onto its territory near oil and gas fields in the Gulf.
"We think that any differences can be solved in an environment of cooperation and with a spirit of partnership and understanding," Mehmanparast added.
The Saudi ambassador to the United Nations, Abdullah al-Mualimi, was quoted in the daily newspaper Okaz last week as saying he had sent a letter to U.N. Secretary General Ban Ki-moon that "details Iran's breaches of the official conventions and treaties between it and Saudi Arabia".
The letter said two Iranian navy boats had intercepted a vessel belonging to state-owned oil giant Saudi Aramco, and that Iranian helicopters flew several times over a gas field at Hasba.
Mehmanparast said it was the Saudis who were at fault.
"The violation that has taken place was on the part of Saudi companies and if this issue is to be followed up, they must be questioned," he said. "The discussion about water border limits between Iran and Saudi Arabia is subject to international laws and documents between the two countries."
This month, Iran impounded a Saudi Arabian fishing vessel that entered its southern territorial waters, according to Iran's official English-language Press TV.
Last month, Saudi border guards arrested 15 Iranians who tried to enter Saudi Arabia by boat, Saudi media reported.
Iran has had disputes with other Gulf neighbours as well.
Both Tehran and the United Arab Emirates claim as their own the three islands of Abu Musa and Greater and Lesser Tunb, which sit near vital oil shipping channels at the mouth of the strategic Strait of Hormuz.
Iran has said its sovereignty over the islands is non-negotiable. Mehmanparast was quoted last month as warning that Iran would consier downgrading ties with the UAE if it continued to make the claims, although state television later denied Tehran was considering such a move.
U.S.-backed Saudi Arabia, the world's No. 1 oil producer, supports the mainly Sunni Muslim rebels fighting to topple Syrian President Bashar al-Assad. Iran backs the Syrian leader, whose Alawite religion is derived from Shi'ite Islam.