India's capital markets regulator altered part of its takeover rules on Tuesday, which could make it harder for Indian firms to use a stock swap for an overseas merger, but said the changes would not apply retroactively.

The changes were announced as the country's leading telecoms firm, Bharti Airtel (BRTI.BO), tries to conclude a $24 billion tie-up before Sept. 30 with South Africa's MTN (MTNJ.J) that would see both firms take stakes in the other.

Under the amendments announced on Tuesday by the Securities and Exchange Board of India (SEBI), depositary receipt holders with voting rights would need to make an open offer once their holding crossed 15 percent in an Indian firm.

Earlier, an open offer was triggered only on conversion of depositary receipts into shares with voting rights.

"The amendment will be applicable only from the day it exists," SEBI chairman C.B. Bhave said on Tuesday.

In July, SEBI had said MTN would need to make an open offer for Bharti only if GDRs were converted by the South African firm and its shareholders to local shares with voting rights.

Under the proposed deal, MTN and its shareholders would receive global depository receipts in Bharti and have a 36 percent economic interest in the Indian firm. Bharti would hold 49 percent of MTN.

Bharti said on Tuesday the structure of the proposed deal with MTN would be fully compliant with the laws in both India and South Africa. [ID:nDEL347844]

Bhave also said SEBI had not received an application for a dual listing, seen as a hurdle that may hold up the tie-up talks.

South African President Jacob Zuma said last week the deal was being stalled by legal problems in India, and media reports have said South Africa is pressing India for an agreement on dual listing of the companies. [ID:nLH668208]

"We have not got any application from anyone that they want to do dual listing," Bhave told a media conference on Tuesday.

"As far as what will be applicable to what deal, it will depend on what circumstances of that deal are."

Bhave told reporters it was "too premature" to comment on the issue of dual listings.