DETROIT -- Tesla CEO Elon Musk sold another chunk of his stock on Thursday after pledging on Twitter to liquidate 10% of his holdings in the electric car maker.

       Musk sold about 640,000 shares for roughly $687.3 million, according to two filings with the U.S. Securities and Exchange Commission. So far this week, the eccentric CEO has sold about 5.1 million shares worth about $5.69 billion.

       About $1.1 billion of the proceeds will go toward paying tax obligations for stock options granted to Musk in September. The options are part of a compensation package Musk and Tesla agreed to in 2018 that provided the CEO with options if Tesla hit certain financial targets. The company is now profitable and worth more than $1 trillion.

       Filings from Wednesday disclosed that Musk exercised options to buy just over 2.1 million shares for $6.24 each. Wedbush Analyst Daniel Ives has calculated that Musk has more than $10 billion in taxes coming due on stock options that vest next summer.

       Musk's sales so far amount to just over 3% of his stake in Tesla. He still owns about 167.5 million shares, or just under 17% of the company, according to the SEC filings and data provider FactSet.

       Last weekend, Musk said he would sell 10% of his holdings in the company, based on the results of a poll he conducted on Twitter. The sale tweets prompted heavy selling in the stock Monday and Tuesday. Shares have fallen for four out of five days this week, and they fell another 3.5% Friday.

       While the stock has slid 16% this week, over the last 12 months it's up 150%.

       Musk is Tesla's largest shareholder and is the wealthiest person in the world, according to Forbes, with a net worth of around $278.7 billion.

       Tesla Inc. is based in Palo Alto, California, although Musk has announced the company will move its headquarters to Texas.