New Delhi, March 13 -- Iran's parliament speaker warned on Thursday that attacks on the Persian Gulf islands that form Iran's southern maritime frontier would provoke a new level of retaliation, underscoring how central they are to the country's economy and security.

In a social media post, Mohammad Bagher Qalibaf said Iran "will abandon all restraint" if the islands come under attack and said U.S. President Donald Trump will be responsible for "the blood of American soldiers."

Although they account for only a small share of Iran's territory, the islands carry outsized importance because of their oil facilities and strategic location.

It was not clear what prompted Qalibaf's remarks, but he was not the first to raise the possibility of...