SHANGHAI, March 15 (Reuters) - China stocks slumped to 21-month lows on Tuesday while mainland firms listed in Hong Kong plumbed their 2008 lows, as surging COVID-19 cases threatened the outlook for the world's second-largest economy and as the central bank dashed expectations for a cut in a key lending rate.

The Ukraine crisis also weighed on sentiment, reviving worries about widening differences between Beijing and Washington as the United States raised concerns about China's alignment with Russia, and prompting global investors to dump Chinese offshore-listed stocks, analysts said.

China's yuan weakened against the dollar for a fourth straight session to hit a three-month low, amid signs of capital outflows on economic slowdown worries and rising geopolitical risks Beijing is facing.

The blue-chip CSI300 index fell 4.6% to 3,983.81, the lowest since June 15, 2020, while the Shanghai Composite Index lost 5% to 3,063.97.

Falls were exacerbated by heavy selling by foreign investors through China's Stock Connect programme. Refinitiv data showed outflows for the seventh straight session, totalling 13.3 billion yuan ($2.08 billion) on the day.,

The People's Bank of China (PBOC) said it would keep the rate on its one-year medium-term lending facility loans unchanged at 2.85% from the previous operation, defying expectations for a cut.

The selling in equities came despite data showing China's economy perked up in the first two months of 2022, with key indicators all exceeding analysts' expectations.

The data "shows the government's supportive policies have started to help the economy," said Zhiwei Zhang, chief economist at Pinpoint Asset Management. "But the macro outlook in the next few months remains challenging. I am surprised the PBOC did not cut the MLF interest rate today."

Adding to the gloom in markets was mainland China's steep jump in daily COVID infections, with new symptomatic cases on Tuesday more than doubling from a day earlier to a two-year high.

China's southern technology hub of Shenzhen suspended public transport including buses and subways, prompting manufacturers such as Apple suppliers Foxconn and Unimicron Technology Corp to halt operations. The financial hub of Shanghai locked down some housing and office compounds.

"China's economy could be severely hit again," Nomura analysts said in a note. "With the much-worsening pandemic and Beijing's resolution in maintaining its zero-COVID strategy, we believe China's 'around 5.5%' GDP growth target this year is becoming increasingly unrealistic."

U.S. national security adviser Jake Sullivan raised concerns on Monday about China's alignment with Russia in a seven-hour meeting with Chinese diplomat Yang Jiechi. Washington has warned of isolation and penalties for Beijing if it helps Moscow in its invasion of Ukraine.

The Hang Seng index fell 5.7% to 18,415.08, the lowest since Feb. 12, 2016. The China Enterprises Index lost 6.6% to 6,123.94, the lowest since Oct. 29, 2008.

The Hang Seng benchmark marked its worst day since July 2015 in the decade's busiest trading day.

"Too much (is) going on in Hong Kong. From what I can tell, the mood there is dire. The exodus of expats is growing every day. Any excuse to sell is the name of the game," Jim Walker, chief economist at Aletheia Capital, told the Reuters Global Markets Forum on Tuesday.

JPMorgan Chase & Co downgraded 28 Chinese stocks listed in the United States and Hong Kong on Monday, sending the tech giants listed in Hong Kong tumbling more than 8% on Tuesday.

"We find China Internet unattractive on a 6-12 month view with an unpredictable share price outlook, depending on the market perception of China's geopolitical risks, macro recovery and internet regulation risk," said JPMorgan Chase & Co in a note.

The Hang Seng Tech Index has lost roughly 22% since last Friday, as the U.S. Securities Exchange Commission identified Chinese companies that will be delisted if they do not provide access to audit documents.

"As the Russia-Ukraine conflict continues, we believe global investors are increasingly nervous about geopolitical risks to China as more and more country and corporates impose sanctions on Russia," JPMorgan Chase & Co said.

(Reporting by Shanghai Newsroom; Editing by Sherry Jacob-Phillips, Sam Holmes, Muralikumar Anantharaman and Subhranshu Sahu)