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* Forex ops halted for at least three foreign banks -sources
* No reason given for suspension in central bank notices
* Move comes amid growing spread between onshore, offshoreyuan
* Sources say authorities have warned of further action (Adds context, comment from banks)
SHANGHAI, Dec 30 (Reuters) - China's central bank hassuspended at least three foreign banks from conducting someforeign exchange business until the end of March, three sourceswho had seen the suspension notices told Reuters on Wednesday.
Included among the suspended services are liquidation ofspot positions for clients and some other activities related tocross-border, onshore and offshore businesses, the sources said.
The sources, speaking on condition that the banks were notnamed, said the notices sent to the affected foreign banks bythe People's Bank of China (PBOC) gave no reason for thesuspension.
The PBOC did not immediately respond to a request forcomment.
The measure follows a slew of steps taken by the Chinesegovernment to keep the yuan stable since it devalued thecurrency in August. (For a FACTBOX, click )
The spread between the onshore and offshore markets for theyuan, or renminbi, has been growing since the devaluation,making it increasingly difficult for the central bank to manageits currency and stem an outflow of capital from an economy thatis facing its slowest growth in 25 years.
The sources told Reuters that authorities had warned thebanks that if they engaged in lucrative carry trade, takingadvantage of the different exchange rates, the central bankwould move to further block arbitrage channels.
"This is part of the PBOC's expedient means to stabilise theyuan's exchange rate," said an executive at a foreign bankcontacted separately.
The sources said the banks might have been targeted due tothe large scale of their cross-border forex businesses.
An economist at a top government think-tank said the measurewas a temporary bid to curb demand for dollars that has beenstrengthening towards the end of the year as the gap between theonshore and offshore yuan exchange rates widens.
"They hope to ease foreign exchange buying pressure and easedepreciation pressure on the yuan," said the economist whodeclined to be identified by name. "But I don't think theauthorities will take very strong capital control measures, theyare likely to reinforce the existing measures."
The move could also ease pressure on the PBOC for directintervention in offshore markets to support the yuan, which hascontributed to a fall of more than $400 billion in China'sforeign exchange reserves this year.
UNDER SCRUTINY
In common with forex markets worldwide, there are noofficial data on which banks are the most active in tradingforeign exchange in China.
A 2015 Asiamoney survey asking market participants whichbrokers they used named Deutsche Bank as the topforeign forex provider in China, followed by Australia and NewZealand Banking Group, HSBC, Citigroup and BNP Paribas.
Asked if they had received the central bank's suspensionnotice, Citi, Deutsche Bank, HSBC and BNP Paribas declined tocomment. There was no immediate response from ANZ.
Standard Chartered and DBS, which alsoconduct trading in foreign exchange in China, did not respond torequests for comment.
The latest move comes just three months since the PBOCordered banks to closely scrutinise clients' foreign exchangetransactions to prevent illicit cross-border currency arbitragebetween the offshore and onshore yuan.
On Wednesday, the country's foreign exchange regulator alsosaid it would improve its reserve position and contingency plansto curb risks from abnormal cross-border capitalflows.
The yuan has come under renewed pressure since late Novemberamid speculation that Beijing would permit more depreciationafter the International Monetary Fund announced the currency'sadmission into the fund's basket of reserve currencies.
The onshore yuan traded in Shanghai has lost 1.44percent of its value since the end of November, and hasrepeatedly hit 4-1/2 year lows.
The offshore market has traced a similar pattern. The HongKong-traded offshore yuan hit an intraday low of 6.5965on Wednesday morning, its weakest since late September 2011. (Reporting by Shanghai Newsroom; Writing by Lu Jianxin,Nathaniel Taplin and John Ruwitch; Additional reporting byLawrence White and Umesh Desai in Hong Kong, and Kevin Yao inBeijing; Editing by Will Waterman)
