NEW DELHI - India sent half a million free doses of COVID-19 vaccine to Sri Lanka on Thursday, officials said, part of diplomatic efforts by Delhi to deepen ties with its neighbours.

India is one of the world's biggest makers of medicines, and the vaccine produced by AstraZeneca and Oxford University is being produced at the Serum Institute of India, a private firm, in the western city of Pune.

Sri Lankan President Gotabaya Rajapaksa received the vaccine, branded as COVISHIELD, from the Indian ambassador Gopal Baglay at a ceremony at Colombo airport, Gotabaya's office said in a statement.

The island nation of 22 million people will begin inoculations of frontline health workers on Friday from the western province, the presidential office said.

"The stock was donated to Sri Lanka following a request made by the President to Prime Minister of India Narendra Modi," it said in a statement.

China, which has invested millions of dollars in ports and highways in Sri Lanka as part of its Belt and Road Initiative, has also promised to deliver 300,000 vials of vaccine, Lalith Weeratunga, principal advisor to the president, said.

India, which is battling the world's second highest caseload of coronavirus, has given away free vaccines to Bangladesh, Bhutan, Maldives, Nepal, Mauritius, and also Afghanistan once local approvals are secured.

"This humanitarian gesture amid the pandemic showcases India’s commitment to its neighbourhood first policy," a government source said.

(Reporting by Sanjeev Miglani; Additional reporting by Waruna Karunatilake in Colombo; Editing by Jan Harvey) ((sanjeev.miglani@thomsonreuters.com; +91 11 49548038; Reuters Messaging: sanjeev.miglani@thomsonreuters.com@reuters.net))