* Talks to continue, likely to update investors on July 31
* Extension likely due to complex nature of deal
* MTN shares gain (Adds details, quotes, analyst comments)
By Narayanan Somasundaram and Rebecca Harrison
MUMBAI/JOHANNESBURG, July 24 (Reuters) - India's Bharti Airtel and South Africa's MTN are expected to extend exclusive talks about a possible merger by at least two to three weeks, four sources familiar with the deal said on Friday.
Shares in MTN extended gains on the news, gaining 1.8 percent and outpacing Johannesburg blue-chip stocks on hopes that the extension signalled talks were on track to clinch a deal.
The two firms in May agreed to hold exclusive talks till July 31 to finalise a deal that could create the world's third-largest wireless group with more than 200 million subscribers, but may now extend them, given the complexity of the deal, the sources said.
"The talks are firmly on but a lot of complications need to be untangled. We reckon a few days more will help," one source said.
The firms would most likely update investors on the talks on July 31, said the sources, who declined to be named as they were not authorised to speak to the media.
"It's probably good news in the short-term for MTN's share price because it probably enhances the possibility of the deal being done," Jan Meitjes, a telecoms analyst at Gryphon Asset Management said.
"And given the fact that the terms needed to be adjusted for the deal to be done, I do think it enhances the chances for better terms for MTN shareholders."
Several MTN shareholders have said they would only back a tie-up if Bharti pays more for its stake in Africa's biggest mobile operator by subscribers.
Ahead of the news, shares in Bharti, India's top telecoms firm, ended 2.1 percent higher at 415.50 rupees, outperforming a 1 percent gain in the Mumbai market.
FUNDING
The sources said that while both sides would be working hard on the deal up to next week's deadline, it looked unlikely they would be ready to disclose an agreement because of outstanding issues on the deal structure and on regulatory approval.
A Bharti spokesman said: "Beyond what we have already communicated in our press note, we have no further comment to offer at the moment."
An India spokeswoman for Standard Chartered, which is advising Bharti on the deal, said the bank would not comment.
No one at MTN could be immediately reached for comment despite repeated calls to the mobile phones of officials.
One source said that while it was still not a done deal, the chances of reaching a final agreement were higher than they were when the companies first said talks had been revived.
Bharti has held talks with banks including Standard Chartered, JPMorgan, Barclays and State Bank of India to raise about $4 billion, sources have said. MTN has held talks with a group of lenders to secure a $3.5 billion syndicated loan for the merger, banking sources have said.
Under the planned deal, Bharti would pay cash and shares to end up with 49 percent of MTN, after MTN pays cash and stock for an effective 36 percent stake in the Indian firm..
The initial stake-swapping deal could eventually lead to a full merger, creating the world's No. 3 mobile phone operator, the companies have said.
The Indian capital regulator earlier this month said MTN could buy 36 percent in Bharti through Global Depositary Receipts without a triggering a mandatory open offer. An open offer was needed only when the GDRs were converted into equity shares with voting rights..
Analysts have said rising competition in India would pressure Bharti to thrash out terms of the merger.
JPMorgan analysts said falling average revenue per user -- a key measure of growth -- in April-June highlighted stiff competitive pressure for Bharti. The firm reported June quarter results on Thursday.
"The performance would lend further credibility to the thesis that Bharti is pursuing MTN as it sees difficult operating environment in India," the analysts said in the note. (Additional reporting by Devidutta Tripathy in NEW DELHI, Gugulakhe Lourie in JOHANNESBURG) (Editing by Rupert Winchester)