The biggest sell-off in U.S. stocks in two years is just getting started, strategists at two of Wall Street’s biggest banks are warning.

While the S&P 500’s average correction has lasted for four months, this pullback is likely to be longer because of the lingering uncertainty over the coronavirus and the constraint facing Federal Reserve policy makers, according to Christian Mueller-Glissmann and Alessio Rizzi at Goldman Sachs Group Inc. That means the retreat that started last Thursday might not find a bottom until at least July.

At Citigroup Inc., strategists led by Jeremy Hale are also leery, citing a lack of clarity on both the virus outbreak and Fed monetary policy. To illustrate where risky assets may become attractive, they pointed to the S&P 500 falling to 2,730, 10% below its 200-day moving average. Reaching that level would require the index to drop an additional 12% from its current level. That would put it on the edge of a 20% bear market decline when measured from the Feb. 19 record.

“We are reluctant to buy the dip in risk assets just yet,” Hale wrote in a note to clients. “It will likely take a further tightening in financial conditions to force the Fed’s hand into acting,” he said. “We await signs of a policy response to pull the trigger on risk.”

Recent remarks from Fed policy makers showed a continued plan to taper Treasury purchases throughout the second quarter, a stance that Hale and his colleagues say has added to investors’ angst. The strategists pointed out that a measure of financial conditions has tightened less than half the magnitude seen amid the rout in late 2018, which ultimately forced the central bank to reverse its stance and cut interest rates. That indicates that the market stress has to worsen before the Fed takes action, they said.

Goldman’s strategists are similarly cautious.

“Without more clarity on a stabilization of global growth, it seems too early to add risk tactically,” Mueller-Glissmann and Rizzi wrote in a note to clients. “With global growth still weak, the shock from the coronavirus lingering and less scope for monetary and fiscal easing, the risk of a more prolonged drawdown remains.”

U.S. stocks erased earlier gains Wednesday amid a barrage of reports on the widening coronavirus outbreak. The S&P 500 fell for a fifth day, extending its loss over the stretch to 8%.

--With assistance from Aoyon Ashraf.

This article was provided by Bloomberg News.