Gulf Cooperation Council secretary general Abdurrahman al-Attiyah "strongly denounced Iran's opening of two administrative offices on Abu Musa island, which belongs to the UAE and is occupied by Iran," a GCC statement said.
The Iranian measure constitutes an "illegitimate action on an indivisible part of the UAE," he said, calling for it to be revoked.
The Riyadh-based pro-Western GCC groups Bahrain, Kuwait, Oman, Qatar and Saudi Arabia alongside the UAE.
Attiyah renewed the GCC's support for the UAE's "full sovereignty" over Abu Musa as well as over Greater Tunb and Lesser Tunb — two other strategic Gulf islands controlled by Iran and claimed by Abu Dhabi.
He also urged the Islamic republic to "respond to the UAE's sincere and repeated calls for resolving the issue through direct negotiations or recourse to the International Court of Justice," the statement said.
The Emirates, backed by other Arab states, has consistently proposed settling the dispute through direct talks or by resorting to the ICJ or international arbitration, but Iran has always refused.
The UAE said on Thursday it had summoned Iran's charge d'affaires in Abu Dhabi and handed him a protest note after Iranian state television reported that Tehran has established a maritime rescue office and a ship registration office on Abu Musa.
An Emirati foreign ministry official described the establishment of the offices as "a flagrant violation" of a November 1971 memorandum of understanding governing the island's status.
Iran, then ruled by the pro-Western shah, gained control of Greater Tunb, Lesser Tunb and Abu Musa as British forces left the Gulf in 1971.
Iran took possession of Greater and Lesser Tunbs, while Abu Musa — the only inhabited island — was placed under joint administration under a deal with Sharjah, now part of the UAE.
But since then, the UAE says, the Iranians have taken control of all access to the island, installed an airport and military base there, as well as encouraged settlers to move in to change its demographic make-up.
The dispute has been a sore point in relations between the UAE and Iran despite wide-ranging links. The oil-rich UAE is Tehran's top trade partner and an estimated 450,000 Iranians live in the country.
