"For the moment I've missed Monte Carlo and Barcelona. And unfortunately I won't be able to be in Madrid either," Nadal said on Instagram.

"The injury still hasn't healed," he added.

The 36-year-old Spaniard has not played since losing in the second round of the Australian Open in January.

Nadal is hoping for a record-extending 15th French Open title at Roland Garros in May, but his hip flexor problem he suffered in Melbourne has badly disrupted his run up to the tournament.

The 22-time Grand Slam singles champion -- he shares the men's record with Novak Djokovic -- had originally thought he would be out for up to two months.

He pulled out of hard-court tournaments at Indian Wells and Miami, with his sights set on a return at the start of the clay-court season, but it did not come to fruition.

"In principle it was supposed to be a recovery period of six to eight weeks and now we're around 14," added Nadal.

"The reality is that the situation is not what we would have hoped for. All medical indications have been followed, but the healing has not gone as they told us initially."

Nadal said he and his team have decided to try a different treatment and see if things improve, but that he was not setting a deadline for his comeback.

His fitness problems have become an increasing issue since he won his 14th French Open crown last year.

At Wimbledon last summer he was forced to withdraw from his semi-final because of an abdominal injury.

The French Open starts on May 28.