Leading U.S. electric car maker Tesla said Monday that it had shut down the assembly line for the Model 3 sedan for four to five days to "improve automation".

This is the second temporary shutdown this year for the model, which is already significantly behind its production schedule.

"During the pause, workers can choose to use vacation days or stay home without pay," Internet media outlet Buzzfeed reported.

On Monday, Tesla's shares closed at 291.21 U.S. dollars, down 3 percent.

Tesla plans to manufacture 5,000 cars per week by the end of the second quarter, which was originally the year-end goal for 2017. "Excessive automation at Tesla was a mistake. To be precise, my mistake. Humans are underrated," Tesla CEO Elon Musk tweeted last week.

The company said two weeks ago that it had achieved a production level of 2,000 Model 3s per week.

Tesla had a similar production pause in February. At the time, the company said the Model 3 production plan includes periods of planned downtime to improve automation and systematically address bottlenecks in order to increase production rates.

"This is not unusual and is in fact common in production ramps like this," the company said in a statement.

The latest pause comes just days after Musk told CBS that he was feeling optimistic about his ability to speed up production and had a "clear understanding of the path out of (production) hell."

During the past month, Tesla has voluntary recalled 123,000 of its older Model S vehicles, dealt with a fatal crash involving its autopilot driving system, and suffered a downgrade of its credit status on Wall Street.

Concerns over the slow pace of Model 3 spurred Moody's Investors Service to cut Tesla's credit rating last month and ask if the company could face a cash crunch this year.

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