Austin, Texas - Elon Musk has again sold several billion dollars in Tesla shares following his expensive purchase of Twitter.

In the past several days, Musk has sold shares in the electric vehicle manufacturer totaling about $4 billion, according to Tesla corporate documents posted early Wednesday morning by the US Securities and Exchange Commission (SEC).

The reason for Musk's stock sales was not immediately clear.

Musk is the founder and chief executive of Tesla, and shares in the company make up the bulk of his massive fortune. He is considered one of the wealthiest people in the world.

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The tech mogul purchased Twitter for about $44 billion. In addition to his own money, Musk took out $13 billion in loans to finance the deal.

According to media reports, servicing those loans will cost Twitter and Musk about $1 billion per year.

Musk has said that Twitter is currently losing about $4 million each day, and advertising revenue for the firm has collapsed as large companies such as Volkswagen and pharmaceutical giant Pfizer put their ads with the social media platform on hold.

Advertisers are reportedly waiting to see how Twitter under Musk's leadership will deal with offensive content such as hate speech, targeted false claims, and conspiracy theories.

Elon Musk sold $8.5 billion in Tesla stock in April and another $7 billion in August, reportedly to fund his takeover of Twitter.