AltaGas Ltd. said Jan. 12 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. (NYSE: WGL), the parent of natural-gas utility Washington Gas.

Shares of WGL, which had a market value of $4.11 billion, closed up 5.9% at $80.26 in regular trading.

AltaGas's shares ended down 1.3% at C$33.65. The company had a market value of C$5.59 billion (US$4.25 billion) as of close on Jan. 12.

Calgary, Alberta-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 U.S. and Canada.

A deal could be announced this month assuming that the talks do not fall apart, or see another bidder, the Journal said.

AltaGas said no agreement had been reached and there was no guarantee that the talks would continue.

A 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, Bloomberg reported in November. The company provides natural gas services in the District of Columbia, Maryland and Virginia. (US$1 = C$1.31)