A total of 21 US service members reported “minor injuries” as a result of drone and rocket attacks on coalition military bases in Iraq and Syria last week, according to the Pentagon.

“Between Oct. 17-18 (ET), 21 US personnel received minor injuries due to drone attacks at Al Assad Airbase, Iraq, and Al-Tanf Garrison, Syria,” Pentagon spokesman Brig. Gen. Patrick Ryder said Wednesday. “All members returned to duty.”

Defense officials told CNN earlier Wednesday that while all of the personnel have since returned to duty, several continue to be monitored for any additional side effects or injuries. The number of injured personnel has risen as more US troops have reported symptoms in the days following the attacks.

“It is important to note, in some cases, service members may report injuries such as (traumatic brain injury) several days after attacks occur, so numbers may change. We will continue to work closely with US Central Command to provide updates as appropriate,” Ryder said.

CNN previously reported that multiple troops sustained minor injuries from the attacks, though the exact number was unclear.

Ryder said Tuesday that US and coalition forces have been attacked at least 10 separate times in Iraq, and three separate times in Syria since October 17, via a mix of one-way attack drones and rockets. US officials have attributed the attacks to Iranian proxy groups operating in the region and have warned of a potential for significant escalation by these groups in the near term.

NBC News was first to report the number of minor injuries in Syria and Iraq.

Officials told CNN earlier this week that at this point, Iran appears to be encouraging the groups rather than explicitly directing them. One official said Iran is providing guidance to the militia groups that they will not be punished – by not getting resupplied with weaponry, for example – if they continue to attack US or Israeli targets.

The attacks have ramped up amid the US’ support for Israel in its war against Hamas and intensified following a hospital blast in Gaza that Palestinian militants and Israel have blamed on each other. US intelligence officials said on Tuesday that the explosion happened when a rocket launched by a Palestinian militant group broke apart in midair and the warhead fell on the hospital.

Iran supports a number of proxy militia groups in countries across the region through the IRGC-Quds Force, and Tehran does not always exert perfect command and control over these groups. How willing those groups are to act independently is a “persistent intelligence gap,” noted one source.

But a senior defense official said the US believes that the proxies are being funded, armed, equipped and trained by Iran, and the US therefore holds Tehran responsible for their actions.

Officials across the administration have reiterated in recent days that the US is preparing for a potential escalation, preparing both defense and offensive capabilities should it become necessary to respond.

The US has around 2,500 troops in Iraq and around 900 in Syria as part of the anti-ISIS coalition, and Secretary of Defense Lloyd Austin said in a statement last weekend that he was deploying additional air defense systems to the region in response to the attacks, including a Terminal High Altitude Area Defense missile system and additional Patriot batteries.

Iran warned on Sunday that the situation could escalate. In a conference with his South African counterpart Naledi Pandor in Tehran, Iran’s Foreign Minister Hossein Amir-Abdollahian said that the Middle East is like a “powder keg,” according to quotes published by state-aligned Tasnim news.

“Any miscalculation in continuing genocide and forced displacement can have serious and bitter consequences, both in the region and for the warmongers,” Abdollahian said, referring to the US and Israel.

The Iranian foreign minister also warned the US and Israel that “if crimes against humanity do not stop immediately, there is the possibility at any moment that the region will go out of control.”