New Delhi/Thimphu, April 28 (IANS) The Indian woman diplomat, held on charges of spying for Pakistan's Inter-Services Intelligence, continued to be interrogated Wednesday and investigations so far indicate she may have passed on sensitive information about New Delhi's plans in Afghanistan. India said the case of spying was a "serious issue".

Senior external affairs ministry officials have been briefed about Gupta's alleged activities in Pakistan, which included supplying classified documents related to Indian interests in Pakistan and Afghanistan, reliable sources said.

The sources said she had passed on information about India's strategy for the Feb 25 foreign-secretary level talks with Pakistan.

Gupta worked as second secretary in the Indian high commission in Islamabad till she was arrested on charges of spying for the Inter-Services Intelligence (ISI).

As the spying scandal surfaced ahead of a meeting between Prime Minister Manmohan Singh and his Pakistani counterpart Yousaf Raza Gilani in Thimphu Thursday, Pakistan reiterated that it was an "internal matter" of India.

The Indian government chose to tread cautiously when asked about the spying incident.

"Investigations are going on. We will have to wait till the investigations are complete and then find out what was the motive and what was the modus operandi," External Affairs Minister S.N. Krishna told reporters in the Bhutanese capital.

Regional leaders from eight countries have gathered in Thimphu for the two-day South Asian Association for Regional Cooperation (SAARC) summit.

"It is a serious issue. That's the reason why she was called back and is now under police custody," he added, a day after Gupta was remanded in police custody.

"MHA (ministry of home affairs) is doing the investigation. Once we get all the reports, what needs to be done will be done by the government of India," the minister said.

A promotee officer of the external effairs ministry, 53-year-old Gupta was arrested five days ago after she was summoned to Delhi on the pretext of consultations for the April 28-29 SAARC summit in Thimphu, official sources said.

Gupta was presented again before a court Monday which remanded her in police custody for five more days.

Gupta was not a member of the main Indian Foreign Service (IFS), but belonged to the lower cadre known as IFS-B.

It's not clear yet what was Gupta's modus operandi or her motive behind spying for Pakistan.

According to a report by CNN-IBN, Gupta disclosed to the ISI the identities of Indian intelligence officers posted as diplomats in Islamabad. Gupta has also mentioned names of intelligence officers posted in some Indian embassies in South Asia and Middle East, government sources told CNN-IBN.

Gupta had been under surveillance for the last four months and action was taken after it was confirmed that she was passing sensitive information to her handlers, sources said.

Gupta worked with the Indian Council of World Affairs (ICWA), a foreign policy think tank, before she was posted at the Indian high commission in Islamabad. She worked there in 2006-2007 as assistant director, sources told IANS. The sources said she specialised in Urdu and was very keen to go to Pakistan.

Authorities are, however, trying to verify whether Gupta was used to "bug" the Indian high commission in Islamabad.