LONDON (Reuters) - Merchant ships and seafarers are increasingly in peril at sea as attacks escalate in the Middle East and the United Nations must do more to protect supply chains, the industry said in a letter released on Friday.

Tensions have soared across the Middle East since the start of Israel's campaign in Gaza in October, with Israel or its ally the United States clashing repeatedly with Iranian-aligned groups in Lebanon, Syria, Iraq and Yemen.

In a letter sent on Thursday to U.N. Secretary-General Antonio Guterres, the world's leading shipping industry associations said Iran's seizure on April 13 of the MSC Aries container ship 50 nautical miles off the United Arab Emirates coast "once again highlighted the intolerable situation where shipping has become a target".

"Innocent seafarers have been killed, seafarers are being held hostage," the letter said.

"The world would be outraged if four airliners were seized and held hostage with innocent souls onboard. Regrettably, there does not seem to be the same response or concern (for ships an their crew members)."

An Indian woman who was a mariner on the MSC Aries returned to the country, India's foreign ministry said on Thursday, adding it was in touch with the other 16 Indian crew members still being held aboard the vessel.

The industry letter said "seafarers and the maritime sector are neutral and must not be politicised".

"Given the continually evolving and severe threat profile within the area, we call on you for enhanced co-ordinated military presence, missions and patrols in the region, to protect our seafarers against any further possible aggression," the letter added.

U.N. officials did not immediately respond to a request for comment.

Recent attacks on merchant shipping in the Red Sea and the Gulf of Aden by Yemen's Iran-aligned Houthis have also affected the global maritime transport chain.

Iran has also seized other vessels in international waters in recent years, heightening risks for merchant shipping in the area.