RABAT, May 9 (Reuters) - U.S.-based Human Rights Watch (HRW) urged Morocco on Friday to investigate media reports that its navy had sunk a boat carrying illegal migrants, causing up to 33 people to drown in the Mediterranean Sea.

The Moroccan government has denied the allegations, saying the boat capsized of its own accord and only 10 people drowned in the April 28 incident off Al Hoceima on its northern coast.

Spanish and Moroccan newspapers quoted survivors as saying that at least one Moroccan navy sailor used his bayonet to puncture the inflatable Zodiac-type boat, crowded with about 70 illegal migrants, after it ignored orders to stop.

HRW cited a survivor's account to Spain's El Pais newspaper: "We begged them to look at our babies and children. The soldier stabbed the boat and just watched with his arms crossed."

The boat was sailing to Spain when the incident occurred about 30 minutes after it left the Moroccan coast, newspapers have said. "Between 28 and 33 passengers, including four children, drowned before another Moroccan naval patrol boat came and rescued the remaining 40 passengers," HRW said.

In a statement, it added: "Moroccan authorities need to investigate properly to find out what really happened. If Moroccan sailors caused the boat to sink and allowed its passengers to drown, they should be prosecuted."

Moroccan Arabic-language daily Al Ahdath al Maghribia on Friday quoted two survivors as saying that four soldiers punctured the boat with their bayonets, ignoring mercy pleas.

"More than half of the migrants who were on the boat perished in the drowning incident which took place in an area located in the middle of Moroccan and Spanish territorial waters," the newspaper wrote.

Under pressure from Spain, Morocco has stepped up its efforts since 2006 to stem the flow of migrants through its territory, deploying more than 20,000 soldiers and policemen, officials say.

About 100,000 migrants a year sail illegally across the Mediterranean to try to reach Europe in the hope of a better life, HRW said, and an estimated 10,000 have died over the past 10 years.

(Reporting by Lamine Ghanmi; editing by Mark Trevelyan)

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