National Broadcasting Co. has announced plans to purchase a cable television channel from Tele-Communications Inc. and to launch two new cable services specializing in financial news and sports. Terms were not disclosed.

The move, disclosed Monday night, marks the NBC television network's first entry into the cable television arena.NBC said it has signed a letter of intent to acquire Tele-Communications's interest in Tempo Television, a cable-television service with 12 million subscribers.

NBC is a subsidiary of General Electric Co., whose stock was down 12.5 cents to $40.875 a share at midday Tuesday on the New York Stock Exchange.

Under the agreement, NBC would offer two new programming services on Tempo Television and Tele-Communications would offer the services to its cable systems and affiliates serving 8 million subscribers.

NBC said it expects both services to premiere in late 1988 or early 1989.

The business news and financial information service would provide coverage through on the day on markets, business trends and the economy in general.

"Using the talent and experience of NBC News, this exciting new service would be aimed at capturing the full spectrum of viewers who are interested in staying up-to-date on the numerous forces and issues that affect their money and financial security," the announcement said.

The sports service, to be aired weekday evenings and on weekends, would provide coverage of sports events around the world, interviews with leading sports figures, sports-oriented talks shows and magazine-format sports shows.

NBC, Tele-Communications and Tempo expect to sign final agreements by the end of this month.