The slowdown in US dealmaking since 2015 is cause for concern, Citi’s equity strategists say.

“In some respects, one of the scariest charts to look at currently is the number of announced mergers & acquisition deals over the past year or two,” Tobias Levkovich, the chief US equity strategist at Citi, said in a note on Friday.

“M&A lawyers argue the ‘uncertainty’ factor, which has come about recently, given some unpredictable aspects of the new Trump administration, has been the issue. It only may explain the last six months, but the trend has been poor for about two years or more. In the past, there has been some correlation with the S&P 500 and thus it could generate more legitimate fears than some of the other excuses that are put forth for not wanting to buy American equities.”

This year through June 5, 7,561 deals were announced, the lowest count since 2013, according to S&P Global Market Intelligence.

M&A volume reached a record $2.055 trillion that year, the firm's data show, slipping in 2016 to $1.7 trillion.

More dealmaking signals, in part, that companies are placing big bets on the long-term growth of certain pockets of the market.

Levkovich said tough antitrust measures from European authorities and the Department of Justice antitrust division may be slowing dealmaking.

Though the chart is worrying, there's potential for a rebound in dealmaking this year, Levkovich said.

"With improved corporate optimism and the narrowing of high yield spreads in 2016, after having widened in 2015 due to problems in the oil patch, the stage looks set for more deals to get done."

This year, $1.12 trillion in mergers and acquisitions has been reported on or officially announced, according to Bloomberg. The reported tie-up between Verizon and Charter Communications would be the most valuable deal this year if it's confirmed.