* 2nd-qtr N. America sales jump 83 pct
* N.America rig count "plateauing"
* Shares fall as much as 4.2 pct (Adds analyst comment; updates shares)
July 24 (Reuters) - Halliburton Co reported impressive second quarter results on Monday but warned the growth in North American rig count was "showing signs of plateauing", sending shares down as much as 4.2 percent.
Oilfield services providers such as Halliburton have been helped by a boom in North American drilling even though oil has stayed below $50.
U.S. rig count rose to 764 in the week ending July 21, compared with 371 rigs in the same week a year ago.
"Today, rig count growth is showing signs of plateauing, and customers are tapping the brakes," said Dave Lesar, Executive Chairman.
"This tapping of the brakes is happening all over the place in North America."
Earlier this month, senior vice president for global business development and marketing at Halliburton, Mark Richard, told Reuters the U.S. shale drilling boom is likely to ease next year as demand on the industry's service sector is unsustainable.
Richard expected rig count to rise to 1,000 by the end of the year, but not beyond that.
"Halliburton didn't necessarily break new ground, they just addressed the elephant in the room," Barclays analyst David Anderson said.
"It took the sentiment down a notch and stock prices are reflecting that."
On Monday, Halliburton swung to a quarterly profit and handily beat analysts' expectations on higher demand in North America.
The company's revenue from North America surged 83 percent to $2.77 billion in the second quarter, due to increased demand for pumping and well-construction services in the region.
Halliburton said margins in its biggest market by sales grew "into double digits" in the quarter and CEO Jeff Miller expected margins from the region to improve.
Barclays' Anderson, however, said earnings of oilfield services companies could peak in 2019, if not by next year.
Net profit attributable to Halliburton was $28 million, or 3 cents per share, in the second quarter ended June 30, compared with a loss of $3.21 billion, or $3.73 per share, a year earlier.
The year-ago quarter included a $3.5 billion charge related to the termination of the Baker Hughes deal.
Halliburton's revenue rose 29 percent to $4.96 billion, marking the company's second straight quarter of increase, after seven quarters of double-digit percentage declines.
Excluding items, the company earned 27 cents per share in the reported quarter, while analysts expected a profit of 18 cents, according to Thomson Reuters I/B/E/S.
Halliburton's shares were down at $42.54 on Monday. Those of bigger rival Schlumberger was marginally lower. (Reporting by Nivedita Bhattacharjee and Yashaswini Swamynathan in Bengaluru; Liz Hampton in Houston; Editing by Arun Koyyur)