The top U.S. diplomat, without presenting solid evidence to support his claim, added that the assessment was "based on intelligence, the weapons used, the level of expertise needed to execute the operation" and other issues.
Iran's Foreign Minister Mohammad Javad Zarif said earlier in the day that the attacks on two oil tankers were "suspicious," noting that they occurred while Japanese Prime Minister Shinzo Abe was meeting with Iran's Supreme Leader Ayatollah Ali Khamenei "for extensive and friendly talks".
Two oil tankers were hit in the Sea of Oman on Thursday morning, with at least one of them operated by a Japanese company. The attacks came amid Abe's visit to Tehran, who is seeking to help ease tensions between Iran and the United States.
"While I very much appreciate P.M. Abe going to Iran to meet with Ayatollah Ali Khamenei, I personally feel that it is too soon to even think about making a deal. They are not ready, and neither are we!" Trump tweeted on Thursday afternoon.
Over the recent weeks, Washington has ratcheted up pressure on Tehran with a series of sanctions, designations and military threats, trying to press Iran back to negotiations after U.S. exit from the landmark Iran Nuclear Deal over a year ago.
Iranian authorities have stressed that Tehran would not sit for negotiations under Washington's threats or sanction pressures.
In May, four commercial vessels suffered similar sabotage off the coast of the UAE.