"The secretary-general sends his best wishes to President Yar'Adua during his continuing convalescence," spokesperson Martin Nesirky said in a statement on Tuesday.

"At the same time, the secretary general wishes to encourage all Nigerians, their leaders and institutions to continue to support the efforts of acting president Jonathan and the government which he leads to help the country overcome its current challenges," he added in an AFP report.

After 93 days of absence, Yar'Adua returned to the country on Wednesday from Saudi Arabia where he was receiving treatment for an acute heart ailment.

Yar'Adua's return threw the country into confusion and revived concerns of a leadership squabble just two weeks after his vice president Goodluck Jonathan was installed as acting president on February 9.

Since then he has not been seen in public, while official delegations have failed to gain access to the ailing leader.