"All our country's measures on Abu Musa island are completely legal and in accordance with Iran's rights governing this Iranian island," foreign ministry spokesman Hassan Ghashghavi said in a statement.
Ghashghavi was reacting to a statement by the six-nation Gulf Cooperation Council on Tuesday condemning the construction and calling for the closure of the offices on Abu Musa, which is also claimed by the United Arab Emirates.
Ghashghavi condemned the GCC statement as "interference in Iran's internal affairs" and branded the UAE's claims to Abu Musa and the Greater and Lesser Tunb islands as "repetitive, unfounded and rejected."
Tehran's establishment of a maritime rescue office and a ship registration office on Abu Musa island drew a formal protest from the UAE earlier this month.
Iran, then ruled by the Western-backed Shah, gained control of the three islands in 1971, as British forces granted independence to its Gulf protectorates.
Iran took possession of the 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 strategic island, and installed an airport and military base there.
The oil-rich UAE has the full backing of fellow GCC states — Bahrain, Kuwait, Oman, Qatar and Saudi Arabia — in its claim to the islands.
It has repeatedly proposed resolving the dispute through direct negotiations or international arbitration, but Iran has always refused.
