The official told CNN that the administration wanted to make it clear it couldn't rule out retaliation within the next few weeks -- or even months -- given how Iran historically has responded to what it views as acts of aggression against the regime.
Iranian President Hassan Rouhani said Saturday that the United States committed a "grave mistake" in killing Soleimani and that Americans "will face the consequences of this criminal act not only today, but also in the coming years."
Administration officials confirmed the existence but not the number or locations of Iranian proxy actors in the Western Hemisphere -- both inside the US and below the southern border -- and warned of attacks possibly coming from Iranian-trained Lebanese Hezbollah, which has sleeper cells in US and European cities, the source familiar with the briefing said.
Iran could also retaliate by hitting US regional allies inside Iraq and in the Middle East, administration officials said, noting the apparent concerns of unspecified Gulf partners.
But a source familiar with the latest intelligence told CNN that it showed vehicle mounted rockets, known as Grad trucks, and other military weaponry were moving closer to US interests, particularly the Al Asad air base in Iraq.
Other targets of concern included the US air base in Qatar and US interests in Kuwait. The source noted those threats have existed for several months but that the intelligence indicated growing urgency because of how close the trucks were getting to US interests.
There are also indications that Iran has ramped up the readiness of its short and medium range ballistic missile force inside Iran since the death of Soleimani but that does not mean a strike by Iran is imminent, the US official directly familiar with the information told CNN. They added that the US is conducting intense surveillance by satellite and other means to determine how soon missiles -- which are liquid fueled -- might be ready.