(Reuters) - Turkey is a key NATO ally to the United States, but the countries’ bilateral relationship has become quite strained.

The Turkish flag flies at the Embassy of Turkey in Washington, U.S., August 6, 2018. REUTERS/Brian Snyder

Reverberations spread through global markets on Friday after U.S. President Donald Trump imposed higher tariffs on metal imports from Turkey, sending the country's lira currency TRYTOM=D3 deeper into a tailspin.

The fallout has been exacerbated over American Christian pastor Andrew Brunson, who is now under house arrest in Turkey. The Trump administration has demanded his release.

But Brunson is not the only sticking point between the two countries. The following is a look at a few of the issues that have contributed to the deterioration in relations between the United States and Turkey:

FALLOUT OVER HALKBANK & IRAN SANCTIONS CASE

A U.S. court in May sentenced Mehmet Hakan Atilla, a Turkish citizen and banker at Turkey's state-controlled Halkbank HALKB.IS, to 32 months in prison after he was convicted of taking part in a scheme to help Iran evade U.S. sanctions.

Federal prosecutors accused Atilla of conspiring with gold trader Reza Zarrab and others to elude U.S. sanctions using fraudulent gold and food transactions. Zarrab pleaded guilty and testified for the prosecutors.

Zarrab described a scheme that he said included bribes to Turkish government officials and that was carried out with the blessing of Turkish President Tayyip Erdogan.

Erdogan has condemned the case as a political attack on his government.

The United States is also considering a fine against Halkbank for allegedly helping Iran evade U.S. sanctions, drawing additional opprobrium from Ankara.

Turkish diplomats visited Washington this week to address frictions between the NATO allies fueled by Ankara’s detention of U.S. consular staff, as well as Brunson, but no breakthrough was announced.

The team also held a meeting with the U.S. Treasury, which imposed sanctions on Turkey’s justice and interior minister over Brunson’s imprisonment.

TENSIONS OVER MANBIJ, YPG

Turkey is infuriated with the United States over its support for the YPG militia in Syria, which Ankara sees as a terrorist organization. Washington sees the militia as a key ally in the fight against the Islamic State.

With U.S. forces stationed alongside Kurdish YPG fighters in northern Syria, Erdogan warned earlier this year that U.S. soldiers could be caught in the cross-fire of Turkish military action against the militia near the town of Manbij.

In June, the two countries agreed that the YPG would withdraw from Manbij and began coordinated but independent patrols near the city.

Erdogan last week said that he did not expect the joint roadmap with the United States regarding Manbij to be impacted by bilateral tensions.

TURKEY’S FAILED COUP AND EXILED CLERIC

Turkey’s demand the United States extradite Islamic cleric Fethullah Gulen, who Ankara has said orchestrated a failed 2016 military coup against Erdogan is another sore point between the two countries.

U.S. officials have said the courts would require sufficient evidence to extradite the elderly Gulen, who has denied any involvement in the coup.

The attempted coup is at the center of the most recent fallout between Washington and Ankara over the detention of Brunson, a pastor from North Carolina who has lived in Turkey for over 20 years.

Brunson is being held on terrorism charges for allegedly supporting Gulen’s network, blamed by Ankara for the failed coup. He has denied those charges. He faces up to 35 years if found guilty. His appeal to be released from house arrest was rejected.

F-35 VS S-400

Washington is also concerned about a deal made between Turkey and Russia for the purchase of Russian S-400 surface-to-air missile batteries, which are not compatible with NATO’s defenses.

Congress has put up roadblocks over the delivery of advanced F-35 fighter jets following the agreement Ankara made with Moscow.

The Turkish delegation visiting Washington earlier this week to discuss Brunson pushed for the delivery of the F-35s, a U.S. official familiar with both sides’ positions said.

DETENTIONS OF U.S. CONSULAR STAFF

Turkey last year arrested a translator at the U.S. consulate in the southern province of Adana and detained a Drug Enforcement Administration employee in Istanbul.

Turkey and the United States suspended the issuance of visas last year after Washington complained about the detention of two locally-hired consular employees on suspicion of a role in the failed 2016 coup. Both countries resumed issuing visas in December.

Washington also says a third locally employed consular staffer is under house arrest.

WASHINGTON, D.C. STREET BRAWLErdogan was vocal against the U.S. justice system over the indictment of his security personnel, who engaged in a brawl with protesters during Erdogan’s visit to Washington in May 2017 where 11 people were hurt.

Washington’s police chief described it as a “brutal attack” on peaceful protesters.

U.S. prosecutors in March decided to dismiss charges against 11 of 15 members of Erdogan’s security team.

Assault charges are pending against four remaining members.

“This is a complete scandal,” Erdogan told reporters after prayers for the Muslim Eid al-Adha celebration in September 2017. “It is a scandalous sign of how justice works in the United States.”