US imposes sanctions on Turkey over Russia weapons.
The US has issued sanctions against its fellow Nato ally Turkey over its deployment of a Russian-made missile defence system acquired last year.
The US says Russia's S-400 surface-to-air missile system is incompatible with Nato technology and a threat to the Euro-Atlantic alliance.
The sanctions announced by the state department on Monday target Turkey's weapons procurement sector.
The move was swiftly condemned by officials in Turkey and Russia.
The US had already kicked Turkey out of its F-35 fighter jet programme over the purchase of the Russian-made missile system.
What is the US objecting to?
"The United States made clear to Turkey at the highest levels and on numerous occasions that its purchase of the S-400 system would endanger the security of US military technology and personnel and provide substantial funds to Russia's defense sector, as well as Russian access to the Turkish armed forces and defense industry," US Secretary of State Mike Pompeo said in a statement.
"Turkey nevertheless decided to move ahead with the procurement and testing of the S-400, despite the availability of alternative, Nato-interoperable systems to meet its defense requirements," he continued.
"I urge Turkey to resolve the S-400 problem immediately in co-ordination with the United States," he said. "Turkey is a valued ally and an important regional security partner for the United States, and we seek to continue our decades-long history of productive defense-sector co-operation by removing the obstacle of Turkey's S-400 possession as soon as possible."
The sanctions target Turkey's Defence Industries Directorate president Ismail Demir, and three other employees.
They include a ban on US export licences, as well a freeze on any assets that are in US jurisdictions.
What does Turkey say?
Turkey's foreign ministry urged the US "to reconsider this unfair decision as announced today", adding that Turkey "stands ready to address this issue through dialogue and diplomacy in conformity with the spirit of alliance".
The ministry warned that US sanctions "will inevitably negatively impact our relations, and (Turkey) will retaliate in a manner and time it sees appropriate".
Ankara argues that the Russian system was purchased after the US refused to sell US-made Patriot missiles. Turkish officials point to the fact that Greece - another Nato ally - has their own S-300, although that was not purchased directly from Russia.
Russia's Foreign Minister Sergei Lavrov also condemned the US decision as "another manifestation of an arrogant attitude towards international law.
"A manifestation of illegitimate, unilateral coercive measures that the United States has been using for many years, already decades, left and right," Mr Lavrov continued.
How important is Turkey?
Turkey has the second-largest army in Nato, a 30-member military alliance.
It is one of the US's key allies, and is located in a strategic position, sharing borders with Syria, Iraq and Iran.
It has also played an important role in the Syria conflict, providing arms and military support to some rebel groups.
However, it has seen relations deteriorate with some Nato members and the EU, who have accused Mr Erdogan of adopting an increasingly authoritarian style following a failed coup in 2016.
How does the S-400 work?
1. Long-range surveillance radar tracks objects and relays information to command vehicle, which assesses potential targets
2. Target is identified and command vehicle orders missile launch
3. Launch data are sent to the best placed launch vehicle and it releases surface-to-air missiles
4. Engagement radar helps guide missiles towards target (Source: bbcnews)
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