Pakistan, July 18 -- Relatives of the five crew members aboard a Boeing 737 cargo plane that crashed into the Arabian Sea off Pakistan last week are urging an international search effort to find the flight recorders to determine the cause.

Debris from the K2 Airways freighter was recovered shortly after the July 7 crash, but the water in the area is about 3,000 m (9,800 feet) deep.

Finding the "black boxes" would require a costly underwater search likely to need foreign assistance, according to aviation experts familiar with deepwater crashes such as Air France 447 in 2009.

The locator beacons on the 27-year-old plane were designed to transmit pings for only 30 days. Recovering the recorders could show whether a navigation system issue...