(Reuters) - Canadian energy infrastructure company AltaGas Ltd (>> Altagas Ltd) said it would buy U.S.-based WGL Holdings Inc (>> WGL Holdings Inc) in a deal valued at C$8.4 billion ($6.42 billion), the latest company pushing deeper into natural gas distribution.

AltaGas will offer WGL Holdings shareholders $88.25 per share held, a 12 percent premium to the stock's Wednesday close.

WGL Holdings's shares were trading at $81.60 in after-market trading.

WGL Holdings, the parent of natural-gas utility Washington Gas, provides natural gas services in Maryland, Virginia and the District of Columbia.

The company also has a retail energy-marketing business and operates natural-gas distribution facilities.

AltaGas's deal for WGL follows Dominion Resources Inc's (>> Dominion Resources, Inc.) $4.4 billion buyout of Questar Corp last year.

Duke Energy Corp (>> Duke Energy Corp), the largest U.S. power company by generation capacity, said in 2015 it would buy Piedmont Natural Gas Co (>> Piedmont Natural Gas Company, Inc.) for $4.9 billion.

Energy firms are betting on natural gas distribution, given rising demand from homes and businesses, and to help offset shrinking profits at utility businesses due to waning power demand.

Calgary-based AltaGas, which operates in the United States and Canada, has three businesses - natural gas gathering and processing, power generation and utilities that deliver natural gas to homes and businesses.

AltaGas said on Wednesday the deal, which includes the assumption of about C$2.4 billion of debt, would add about 7-9 percent to earnings per common share in the first full year of operations.

The company said it expected the transaction to close by the end of the second quarter of 2018.

The Wall Street Journal reported earlier this month, citing sources, that AltaGas was is in talks to merge with WGL Holdings.

J.P. Morgan Securities LLC is the lead financial adviser to AltaGas, while TD Securities Inc is also providing financial advice to the company.

Goldman Sachs and Lazard are the financial advisers to WGL.

(Reporting by Komal Khettry in Bengaluru; Editing by Saumyadeb Chakrabarty and Sriraj Kalluvila)