Most U.S. stocks closed lower on Monday, extending the recent sell-off in the market, the Hong Kong-based Wande News Agency reported.

The standard & poor's 500 index fell 15.88 points, or 0.4%, to 4008.01, while the technology-heavy Nasdaq index fell 142.21 points, or 1.2%, to 11662.79. However, the Dow Jones industrial average rose 26.76 points, or 0.1%, to 32223.42.

Even after Friday's rally, the S & P 500 has fallen for six consecutive weeks, its longest weekly decline since June 2011. The index is down 16% this year. Investors worry that the Fed will take too aggressive action to deal with inflation, plunging the US economy into recession.

As investors sell US stocks, Buffett's Berkshire Hathaway has seen the market downturn as an opportunity to increase stock spending. The company has invested tens of billions of dollars in the past few months after cash reserves reached an all-time high at the end of 2021.

The Omaha-based company bought 901768 shares in Occidental Petroleum last week, according to regulatory filings. Berkshire began buying Western oil companies at the end of February and became one of its top 10 shareholders.

In the past few months, Berkshire has also increased its stake in Chevron Corp and bet on M & An arbitrage by Activision Blizzard. Berkshire bought Hewlett-Packard, Citigroup Inc and Ally Financial. In addition, the company continues to increase its stake in Apple Inc, which is still the largest shareholding by Apple Inc.

At the same time, Berkshire Hathaway also reduced its position in Wells Fargo & Co. Wells Fargo & Co was once one of Berkshire Hathaway's main stocks and has been part of Berkshire's portfolio since 1989.

Berkshire filed 13F filings with the Securities and Exchange Commission (SEC) on Monday, allowing investors to learn about the changes in its positions over the past few months. SEC requires all institutional investors with assets under management of more than $100m to submit documents within 45 days of the end of each quarter. Because institutions must disclose the shares they hold in the documents, as well as the size and market value of each position, investors usually use 13F to measure the size of fund managers' investments in the stock market.

The conclusion can be drawn from Berkshire's documents that the market turmoil has allowed the company to spend heavily. As a long-time advocate of value investing, Buffett has always advised investors to be "greedy when others are afraid." For most of the past two years, this idea may have been difficult to put into practice, during which investor sentiment has largely gone from fear to fear.

"Cash is dry powder, and he has a lot of cash," Rupal Bhansali, chief investment officer for global equities at Ariel Investments, said of Buffett. Buhansari manages Ariel's global mutual fund, which holds Berkshire shares.

Mr Buhansari and others also believe that Berkshire's investments in Chevron Corp and western oil companies may reflect a bet that commodity prices will remain high for some time.

Energy stocks are the best performers in the S & P 500 so far this year, benefiting from soaring commodity prices. Commodity prices began to soar after geopolitical factors raised concerns about disruptions in oil and gas supplies. Chevron Corp's share price is up 47% this year, while that of western oil companies is up 134%. By contrast, the S & P 500 is down 16%.

Jim Shanahan, a senior equity research analyst at Edward Jones, said energy stocks also have two characteristics that Buffett has traditionally favored: low valuations and shareholder returns in the form of buybacks and dividends. Dividend-paying stocks have outperformed the S & P 500 this year, in part because investors affected by market volatility are looking for stocks that offer stable cash returns.

In addition, Berkshire has stepped up its purchases of bank shares, which tend to be relatively undervalued and offer dividends. The company bought 55 million shares of Citigroup Inc in the first quarter, worth about $3 billion. In 2020, Berkshire sold shares in Goldman Sachs Group, JPMorgan Chase & Co and Wells Fargo & Co, missing the gains in financial stocks in the second half of the year and 2021.

With stock market volatility still high, many investors and analysts expect Buffett and Berkshire's portfolio managers Ted Weschler and Todd Combs to continue to pour cash into the market in the coming months.

Buffett said in February that Berkshire had a lot of cash on hand at the end of last year because it couldn't find a company that seemed worthy of long-term investment. As of March 31, the company had $106.3 billion in cash, down from $146.7 billion at the end of 2021.