By Tommy Wilkes and Amjad Ali

NEW DELHI/ISLAMABAD (Reuters) - India and Pakistan on Thursday announced they would each expel one of the other's diplomats amid growing tension between the nuclear-armed arch-foes over the disputed region of Kashmir.

India said it would expel a Pakistani diplomat based in New Delhi who allegedly ran a spy ring that collected sensitive information about Indian security operations along its border.

Late on Thursday night, Pakistan's foreign ministry said it had declared an Indian diplomat, Surjeet Singh, persona non grata and given him 48 hours to leave the country.

Police in the Indian capital said the Pakistani diplomat was detained on Wednesday outside the gates to Delhi Zoo, where he had met two Indian associates whom police believe he had recruited to spy for him.

The Pakistani diplomat, who reportedly worked in Pakistan High Commission's visa section, and his alleged Indian accomplices were found with forged documents, defense-related maps, deployment charts and lists of officers working along India's border with Pakistan, Indian police said in a statement.

"There was high probability that the information passed on by these anti-national elements to PIO (Pakistan intelligence operative) is being used against the national interests and could be highly detrimental for national security," they said, adding they had been trying to break the spy ring for six months.

An Indian foreign ministry spokesman said the man, who was released from custody under diplomatic immunity rules, must leave the country by Saturday.

Pakistan's High Commission in New Delhi rejected the allegations, saying in a statement it "never engages in any activity that is incompatible with its diplomatic status".

Later on Thursday, Pakistan's foreign ministry announced it had declared Singh persona non grata and informed the Indian High Commission he had until Saturday to leave the country.

The statement said Singh was accused of activities "that were in violation of the Vienna Convention and the established diplomatic norms" but did not elaborate.

An aide to India's prime minister in New Delhi said the government was looking into the matter. India's external affair's ministry spokesman was not immediately available for comment.

India and Pakistan have been at loggerheads since a group of gunmen killed 19 Indian soldiers in September at an army camp in Kashmir, an attack India blamed on Pakistan-based militants.

India said it had sent special commandos into Pakistan-controlled Kashmir to kill militants in a retaliatory operation that sharply soured relations between the neighbors.

Pakistan says the operation never happened and accuses India of inventing it to distract attention from its crackdown on protests in the part of Kashmir it controls.

Indian and Pakistani troops face off against each other along the de facto border in divided Kashmir - a region they both claim in full but control in part - and have exchanged fire several times this week in cross-border shelling.

(Reporting by Tommy Wilkes; Editing by Tom Heneghan and Toby Chopra)