File photo shows Nigerian President Muhammadu Buhari (C) meeting with the released 21 Chibok school girls at the State House in Abuja, Nigeria, Oct. 19, 2016.(Xinhua/Olatunji Obasa)

ABUJA, March 13 (Xinhua) -- Nigerian leader Muhammadu Buhari transmitted a letter to the National Assembly on Monday, formally announcing his resumption of duties after a long medical vacation in the United Kingdom.

"I have resumed my functions as the President of the Federal Republic of Nigeria with effect from Monday, March 13, 2017, after my vacation," the 74-year-old Nigerian leader wrote in the letter sent to the country's Senate and the House of Representatives.

Apart from formally announcing the president's return to the country after the vacation which lasted more than six weeks in London, the letter was written in compliance with Section 145 of Nigeria's amended 1999 constitution, according to a statement released here by presidential spokesman Femi Adesina.

Buhari returned to a warm welcome by Nigerian citizens last Friday and pledged to rededicate himself to the service of the west African nation in an address he delivered few hours after his arrival.

Upon his arrival in Abuja, the Nigerian capital, Buhari, though frail-looking, said his health had improved significantly and expressed gratitude to Nigerians for their prayers and show of goodwill.

However, there may be the need for him to embark on another medical trip, "for further follow up within some weeks," he added.

Buhari had left the country on January 19 for the vacation during which he had a routine medical check-up.

The holiday was extended based on doctors' recommendation for further tests and rest.

Before leaving Nigeria, Buhari had sent a letter to the National Assembly requesting a 10-day vacation. He transferred power to Vice President Yemi Osinbajo as Acting President.

The Nigerian leader could not resume his duties earlier in February when he was expected to return to Nigeria. A letter requesting an extension of his vacation was sent to the Senate. He said he required further medical tests.