Millions of Airtel Nigeria subscribers were without mobile phone coverage this week following a dispute between workers and the company, which is owned by India''s top mobile operator Bharti Airtel .

Nigeria Labour Congress, an umbrella trade union group, said Airtel unfairly sacked thousands of employees, but the mobile company denies the allegations, saying the workers were employed by other companies it contracts.

Airtel did not sack any employee. The employees being referred to work for Tech Mahindra and Spanco, who provide us Call Centre services, the company said in a statement on Thursday.

The employees in question were contracted through two employment agencies, namely HR Indexx and CCSNL, whose contracts with Tech Mahindra and Spanco expired on Sept. 30, 2011.

Airtel said some people claiming to be NLC members shut down 15 showrooms this week and attacked its switchboard in the capital Abuja, cutting off 5 million customers from its network across the north of Africa's most populous nation.

The company said Spanco and Tech Mahindra were working on plans to recruit new employees, which would include some of the contracts that have recently expired.

Bharti entered the Nigerian market last June when it acquired the African telecoms assets of Kuwaiti group Zain in a $9 billion deal, giving it access to what it said was the biggest future continent for telecoms.

Nigeria, with more than 140 million people, is seen by investors as one of the world's leading frontier markets and is increasingly competitive. It has overtaken South Africa as the continent's biggest mobile market.