Concerns increased today, Monday, about the potential collapse of the ceasefire between Washington and Tehran, after the United States announced that it had seized an Iranian cargo ship that attempted to breach the imposed blockade on the Islamic Republic’s ports, which threatened to respond.

The efforts to reach a more lasting peace in the region appeared to be on shaky ground, as Iran announced that it would not participate in a second round of negotiations that the United States had hoped would begin before the ceasefire ends tomorrow, Tuesday.

The United States continued the blockade on Iranian ports, while Iran lifted the blockade it had imposed on maritime navigation through the Strait of Hormuz, through which nearly one-fifth of global oil supplies usually passed, and then re-closed it once again.

On Sunday, the U.S. military said it had fired upon a cargo ship flying the Iranian flag as it sailed toward the Iranian port of Bandar Abbas. U.S. President Donald Trump wrote on social media, “We now have full control of their ship and are checking what’s on board!”

The Iranian military stated that the ship had come from China. State media quoted a military spokesman as saying, “We warn that the armed forces of the Islamic Republic of Iran will soon retaliate for this act of armed piracy committed by the U.S. military.”

Oil prices surged, and stock markets saw volatility as traders anticipated that traffic to and from the Gulf would remain at a minimum.

The war, now in its eighth week, has caused the greatest shock in the history of global energy supplies, driving up oil prices due to the effective closure of the strait.

Thousands were killed due to U.S. and Israeli strikes on Iran and Israeli attacks on Lebanon that occurred alongside the war, which began on February 28. Iran responded to the attacks by launching missiles and drones on Israel and neighboring Arab countries hosting U.S. bases.

Iranian Parliament Speaker Mohammad Bagher Ghalibaf, who led the Iranian negotiating team, had previously said that both sides had made progress but remained far from reaching an agreement on nuclear and strait issues.

European allies, frequently criticized by Trump for not aiding his war efforts, fear that the U.S. negotiating team will push for a quick and superficial agreement that would require months or years of subsequent technically complex talks, according to Reuters.