Iran shot down a Ukrainian jetliner near Tehran, killing all 176 on board, as it was firing ballistic missiles at US troops in Iraq.
Iran will punish all those responsible for the accidental shooting down of a Ukrainian passenger plane, President Hassan Rouhani said in a televised speech on Tuesday, adding that the "tragic event" would be investigated thoroughly.
"It was an unforgivable error ... one person cannot be solely responsible for the plane crash," he said.
"Iranian armed forces admitting their mistake is a good first step ... We should assure people that it will not happen again," he said, adding that his government was "accountable to Iranian and other nations who lost lives in the plane crash".
After his speech, Iran's judiciary said it had arrested some people for their role in shooting down the plane, judiciary spokesman Gholam Hossein Esmaili said on Tuesday.
He did not give further details.
Tensions have soared since the US air strike earlier this month that killed Qasem Soleimani, Iran's top general and the architect of its regional military activities.
Iran responded by firing ballistic missiles at US troops in Iraq, during which it said, it had "accidentally" shot down a Ukrainian jetliner near Tehran, killing all 176 on board.
US warns its vessels transiting through Gulf
Earlier on Tuesday, US warned of threats to commercial vessels in and around the Gulf in the wake of its confrontation with Iran.
“Heightened military activity and increased political tensions in this region continue to pose serious threats to commercial vessels,” the US said in a maritime warning.
“Associated with these threats is a potential for miscalculation or misidentification that could lead to aggressive actions.”
It said vessels transiting the Gulf, the Gulf of Oman and the Strait of Hormuz — through which one-third of all oil traded by sea passes — could encounter GPS interference or communications jamming.
It said some vessels have reported communications from “unknown entities falsely claiming to be US or coalition warships.”
It said ships contacted by Iranian forces should identify themselves and say that they are proceeding in accordance with international law.
It advised ships to refuse to allow Iranian forces to board but not to forcibly resist them, and to immediately contact the US Fifth Fleet.
Iran was accused of sabotaging oil tankers in the Gulf last year. It denied those allegations but acknowledged seizing a British-flagged oil tanker in response to the impounding of an Iranian oil tanker by authorities in Gibraltar.
The Iranian tanker was suspected of intending to violate sanctions to deliver oil to Syria, a close ally of Iran.
Both ships were released weeks later.
The tensions are rooted in President Donald Trump's decision to withdraw the US from Iran's 2015 nuclear deal with world powers and to impose “maximum” economic sanctions.
The sanctions have devastated Iran's economy.