Investigators have extended the search for the flight recorders of an Air France plane that crashed into the Atlantic last month and still hope to find them, France's transport minister said on Friday.

Flight AF 447 from Rio de Janeiro to Paris crashed on June 1, killing all 228 people on board, but investigators have so far failed to pick up any signals emitted by the "black box" recorders. The signals are sent out for at least 30 days. "We could stop around ... July 10, but we won't. If we don't find them with the classic means, we will continue through submarine exploration," Transport Minister Dominique Busseareau told French radio.

He said the chances of finding the black boxes were fairly weak, but they would nevertheless try.

French investigators said on Thursday the plane hit the water intact and at high speed, but was missing for six hours before an emergency was declared.

Evidence from wreckage indicates the plane was broken apart by impact with the water, which it struck facing forwards.