President Donald Trump today promised new sanctions against Iran, while Saudi Arabia presented what it called evidence of Iran’s responsibility for aerial strikes on Saudi oil processing facilities.

At a news conference in Riyadh, the Saudi capital today, the Saudi Defense Ministry presented debris from the site and videos that appeared to be from surveillance cameras on the ground.

“This attack was launched from the north and was unquestionably sponsored by Iran,” said Col. Turki al-Maliki, a spokesman for ministry.

He said Saudi officials were still trying to determine exactly where the strikes originated.

In Iran, which has vehemently denied any involvement, state media reported today that U.S. obstruction might force President Hassan Rouhani to miss a major United Nations gathering next week in New York City.

The attack Saturday, which Saudi officials said involved some two dozen drones and cruise missiles, temporarily cut Saudi oil processing in half, shaking global markets and escalating the already high level of tension between the United States and Iran, raising fears of military clashes and even outright war.

Trump has already imposed punishing economic sanctions on Iran and some of its top officials, in what the administration has described as a “maximum pressure” campaign to force Iran to negotiate new limitations on its nuclear program and stop its sponsorship of militant groups across the Middle East.

This morning, he wrote on Twitter that he had told Treasury Secretary Steven Mnuchin “to substantially increase Sanctions on the country of Iran.” It was not immediately clear how extensive the latest round of penalties would be or whether they would be aimed at Iran generally, specific elements of the regime or individuals.

Iran and its ally, the Houthi rebel faction in Yemen, insist that the Houthis — who are fighting a Saudi-led coalition in Yemen’s civil war — carried out the strikes in retaliation for the extensive bombing by Saudi Arabia that has killed thousands of people in Yemen. The Houthis are known to use weapons supplied by Iran, but the attack showed a level of technological sophistication far beyond what the Houthis had demonstrated before.

United States and Saudi officials have said that the weekend attack clearly used Iranian weapons. The Americans have also said that evidence that has not been made public points to a strike launched from Iran, to the north, not from Yemen, to the south.

“This attack did not originate from Yemen, despite Iran’s best effort to make it appear so,” said al-Maliki, the Saudi spokesman.

He also said that 18 drones hit one site and four cruise missiles hit another, and that three missiles fell short.

It was not clear how the evidence shown by the Saudis indicated that the attack came from the north or did not come from Yemen. Nor did the Saudis make it clear whether they were saying that Iran had the kind of indirect involvement, through supplying munitions and training, that it has had in previous Houthi strikes on Saudi Arabia, or something more direct, like Iranian personnel taking part or the attack’s having been launched from Iran.

The Houthis have launched missiles at Saudi targets before, but none of the attacks had the scale, sophistication or practical effect of the one Saturday.

Trump and Iran’s president, Hassan Rouhani, have been expected to attend the annual U.N. General Assembly session in New York next week, and there was even speculation this summer about a possible face-to-face encounter between them.

But today, the official Islamic Republic News Agency reported that an Iranian advance team had been unable to go to New York to prepare for the meeting because the United States had not granted visas. As a result, it said, Rouhani and his delegation might not attend the gathering, which runs from Tuesday through the following Monday.

Trump has said repeatedly that he is open to a meeting with Rouhani, which would be the first between leaders of the two countries after four decades of antagonism, but Rouhani has said that Iran would not agree until the United States lifted economic sanctions.

Rouhani sent a formal note Monday to the United States denying an Iranian role and warning that any U.S. action against Iran would bring retaliation, Iranian state news media reported today. The note went through Swiss envoys who act as go-betweens because the United States and Iran do not have diplomatic relations.

Secretary of State Mike Pompeo was scheduled to meet today in Saudi Arabia with Crown Prince Mohammed bin Salman, the heir to the throne and the country’s day-to-day ruler, but it was not clear whether there would be any public announcement about the talks. In a statement, the State Department said the two would “discuss the recent attack on the kingdom’s oil facilities and coordinate efforts to counter Iranian aggression in the region.”

Trump tweeted Sunday that the United States was “waiting to hear from the Kingdom as to who they believe was the cause of this attack, and under what terms we would proceed!”

Last year, the president withdrew the United States from the 2015 accord limiting the scope of Iran’s nuclear program and reimposed sanctions that had been lifted as part of the deal. This year, Trump has hit Iran and Iranian officials with new rounds of sanctions.

The main penalties seek to choke off Iran’s international oil sales, the heart of its economy. They bar any company doing business with Iran from using the U.S. banking system, whose reach is so vast that Trump’s actions apply to many overseas businesses.

After Trump began imposing more sanctions this year, several tankers were damaged near the Persian Gulf, and Western governments said they had been sabotaged by Iran, which Tehran denied. Iran has also seized several foreign vessels in or near the Strait of Hormuz, including a British-flagged tanker it has held for two months.

Analysts have described those incidents — and, possibly, the attack on Saudi Arabia — as one prong of a two-pronged strategy to pressure other nations to provide sanctions relief by showing that Iran can interrupt world oil supplies. The other prong, analysts say, is that Iran began exceeding the limits on its nuclear program under the 2015 deal.