The U.S. military lifted its blockade on ships entering and exiting Iranian ports Thursday, U.S. Central Command announced, implementing one of the first conditions of a ceasefire agreement that opens a 60-day window for negotiations between Washington and Tehran.

The move reverses the blockade imposed in April, which had halted Iranian oil exports and effectively shut the Strait of Hormuz to much of the world’s oil traffic. Iran, for its part, has committed to allowing oil tankers to move safely through the strait, where roughly 20% of the world’s oil transited before the conflict began.

The agreement states that Iran will allow commercial vessels to transit the Strait of Hormuz “with no charge for 60 days only,” after which “future administration and maritime services” will be determined by Iran along with Oman and other Persian Gulf states. Iranian officials have suggested they may impose “service fees” on ships, which industry analysts called legally questionable on an international waterway, according to the agreement’s text.

When ships will begin sailing through the strait in significant numbers remains unclear, given the presence of mines laid by Iran that the U.S. and other nations are in the process of clearing. CENTCOM said the U.S. Navy remains in the area to ensure that the terms of the ceasefire are obeyed.

Vice President Vance, who is leading the negotiations with Iran, is expected to head to Switzerland as early as this weekend, though he did not give a set date. He defended the memorandum of understanding signed earlier this week during a press briefing at the White House on Thursday.

Vance said the U.S. holds “all the cards” at the moment, insisting that Iran will not significantly benefit until it can “verify for us that they are changing their behavior.” He called the initial move to lift the blockade and allow Iranian oil through “not a new benefit to the Iranians.”

“They were selling oil for many, many years, well before we ever put the blockade,” Vance said. “We imposed that blockade. They stopped selling oil, and now we’ve lifted the blockade in order to promote the free flow of energy across — across the world.”

The 14-point memorandum gives a broad outline of an agreement but leaves some of the thorniest points of contention yet to be resolved. The document says the ceasefire extends to Lebanon, which would require cooperation from the Israelis, who are not a party to the agreement. It also states plans for the creation of a $300 billion fund “for the reconstruction and economic development of the Islamic Republic of Iran” in coordination with “regional partners,” with details to be sorted within 60 days.

Details of lifting sanctions against Iran and disposing of its stockpiled enriched material are all part of what negotiators hope to resolve over the next two months.