• U.S. stock futures fell Monday to start June as U.S. trade tensions with key trading partners increased.
• China accused the U.S. of undermining an agreement that led both sides to temporarily slash tariff rates, while the European Union said it was prepared to respond to President Donald Trump's plan to raise steel tariffs to 50%.
• Investors are joining Warren Buffett in piling into the short-term U.S. Treasury market.

Here are the most important news items that investors need to start their trading day:

1. May flowers wilt

U.S. stock futures fell Monday to kick off June, following a strong performance for equities in May. The S&P 500 rose 6% for May, its best monthly performance since November 2023. The Nasdaq Composite jumped 9% for the month, while the Dow Jones Industrial Average climbed 4%. Investors will be watching tariff developments after a wave of news in recent days. Tensions between the U.S. and China climbed again, President Donald Trump said he would double duties on steel to 50% and an appeals court paused a ruling that would have scrapped many of his administration's levies on imports. The May jobs report due Friday will offer more clarity on how the U.S. economy has weathered Trump's unpredictable trade policy. Follow live market updates.

2. Pointing fingers

The trade detente between Washington and Beijing appears to be cracking. China accused the U.S. of undermining an agreement that led the trading partners to slash tariff rates for three months. The complaint came days after Trump contended China was violating the terms of the deal. The Trump administration's export restrictions related to semiconductors and decision to revoke visas for Chinese students "seriously undermine" the trade pact, a Chinese commerce department spokesperson said. Trump advisor Kevin Hassett had said over the weekend that the U.S. president and Chinese President Xi Jinping could have a conversation about trade as soon as this week. It is unclear now whether either side will respond by changing their tariff policies.

3. Steel curtain

China was not the only front where trade tensions increased. Trump on Friday threatened to impose the steep 50% tariff on steel imports as he lauded his approval of U.S. Steel's merger with Japan's Nippon. He said the increase would take effect on Wednesday – though he has repeatedly set effective dates for tariffs and then rescinded them. He said the duties would "further secure" the U.S. steel industry. "At 25%, they can sort of get over that fence," Trump said. "At 50%, they can no longer get over the fence." The European Union criticized Trump's move, as a spokesperson said it "undermines" efforts to reach a trade agreement. The bloc is "prepared to impose countermeasures," the spokesperson added.

4. Buffett line

Trade conflict and concerns about U.S. fiscal health have contributed to volatile long-term bond yields. That has made the short end of the Treasury market appealing for investors. Warren Buffett's Berkshire Hathaway reportedly owns 5% of all short-term U.S. Treasuries, and other investors seem to be joining the conglomerate in that move. The iShares 0-3 Month Treasury Bond ETF and SPDR Bloomberg 1-3 Month T-Bill ETF both rank among the top 10 ETFS in investor flows so far this year.  

5. Rarefied air

The business class airline seat is getting even cushier. With perks like privacy and caviar, carriers are competing to draw more high-income travelers. American Airlines will soon debut upgraded business class "suites" that feature sliding doors and a wireless charging pad, among other amenities. The bets on high-end seating come as the industry tries to protect thin profit margins. As demand for domestic coach seats falls, executives hope wealthier travelers will keep shelling out for premium experiences.

– CNBC's Sean Conlon, Anniek Bao, Erin Doherty, Spencer Kimball, Jason Gewirtz and Leslie Josephs contributed to this report.