By Sonam Rai and Nick Carey

(Reuters) - Tesla Inc (TSLA.O) on Wednesday cut U.S. prices for all its vehicles to offset lower green tax credits, and fell short on quarterly deliveries of its mass-market Model 3 sedan, sending shares of the electric vehicle maker down nearly 7 percent on worries of future profitability.

Analysts questioned whether the $2,000 price cut on all models signaled lower demand in the United States, and ultimately whether the move would undermine nascent profitability at the Silicon Valley automaker, which has never posted an annual profit.

"In our view, this move could suggest that what many bulls assume to be a substantial backlog ... for Tesla may be less robust," wrote Bank of America analyst John Murphy in a client note.

Chief Executive Elon Musk, who has often set goals and deadlines that Tesla has failed to meet, surprised investors by delivering on his pledge to make Tesla profitable in the third quarter, for only the third time in its 15-year existence. But the company is unprofitable for the first nine months of 2018, and cash flow remains a concern for investors.

Musk has been under intense pressure to deliver on his promise of stabilizing production for the Model 3, which is deemed crucial for easing a cash crunch and achieving long-term profitability. It said it was churning out almost 1,000 Model 3s daily, broadly in line with Musk's promises but slightly short of Wall Street expectations.

The company said it would begin delivering Model 3s to Europe and China in February.

The price cut of $2,000 beginning on Wednesday on the Model 3 - as well as on its higher-priced Model S and Model X - took the market by surprise and weighed on the stock, pushing it down 6.8 percent to close at $310.12, after falling as much as 10 percent during the session.

The lower price comes as automakers expect U.S. new vehicle sales to weaken in 2019, and amid increased competition from new electric vehicle entrants. Tesla sales benefited from a $7,500 federal tax credit on electric vehicles throughout 2018, but that full credit expired at the end of 2018, and new buyers will now receive only half that amount.

Under a major tax overhaul passed by the Republican-controlled U.S. Congress in 2017, tax credits that lower the cost of electric vehicles are available for the first 200,000 such vehicles sold by an automaker. The tax credit is then reduced by 50 percent every six months until it phases out.

"The price cut is what's driving the stock lower, as it openly acknowledges the sunset of subsidy dollars is a material headwind," said Craig Irwin, an analyst with Roth Capital Partners.