(Adds AltaGas and WGL responses, changes sourcing)
Jan 12 (Reuters) - Canadian energy infrastructure company AltaGas Ltd ALA.TO said on Thursday it was in talks with a third party over a potential transaction.
AltaGas did not name the company, but the Wall Street Journal, citing people familiar with the matter, said the firm was is in talks to merge with WGL Holdings Inc WGL.N , the parent of natural-gas utility Washington Gas.
Shares of WGL, which had a market value of $4.11 billion, closed up 5.9 percent at $80.26 in regular trading.
AltaGas's shares ended down 1.3 percent at C$33.65. The company had a market value of C$5.59 billion ($4.25 billion) as of Thursday close.
A deal could be announced this month assuming that the talks do not fall apart, or see another bidder, the Journal said. (http://on.wsj.com/2jcr9kM)
AltaGas said no agreement had been reached and there was no guarantee that the talks would continue. WGL spokesman said the company did not comment on "market rumors".
WGL was weighing options, including a sale, after receiving takeover interest from Spain's Iberdrola SA IBE.MC , Bloomberg reported in November. (http://reut.rs/2ilgi3H)
Calgary-based AltaGas operates in three segments - natural gas gathering and processing, power generation and utilities that deliver natural gas to homes and businesses. The company has a presence in both the United States and Canada.
WGL provides natural gas services in the District of Columbia, Maryland and Virginia. ($1 = 1.31 Canadian dollars)