Image copyright Getty Images Image caption The US issued more than 8,000 immigration visas to Nigerians in 2018

The US has announced it is expanding its curbs on immigration to include six more countries, including Nigeria, Africa's most populous nation.

Citizens from Nigeria, Eritrea, Sudan, Tanzania, Kyrgyzstan and Myanmar will now be blocked from obtaining certain types of visas.

People from those countries will still be able to visit the US as tourists.

In 2018 the US issued twice as many immigration visas to Nigeria than to the other five nations combined.

An official said the new measures were the result of failures by the six countries to meet US security and information-sharing standards.

"These countries, for the most part, want to be helpful but for a variety of different reasons simply failed to meet those minimum requirements that we laid out," acting Homeland Security Secretary Chad Wolf told reporters on Friday.

He said officials would work with the countries on bolstering their security requirements to help them get off the list.

US President Donald Trump first introduced a travel ban in 2017. It currently closes US borders to citizens from seven countries, most of them with Muslim majorities.

In 2018 the US issued more than 8,000 immigration visas to citizens of Nigeria. That same year, just over 2,000 were issued to Sudanese nationals, 290 to Tanzanians, and just 31 to Eritreans.

The US had previously announced a ban on certain types of visas for Eritreans in 2017.

What are the new rules?

The US said it would suspend the issuance of visas that can lead to permanent residency for nationals of Nigeria, Eritrea, Kyrgyzstan, and Myanmar.

Sudanese and Tanzanian nationals will no longer be allowed to apply for "diversity visas", which are available by lottery for applicants from countries with low rates of immigration to the US.

Mr Wolf said non-immigrant visas given to people for temporary stays - including visitors, those doing business or people seeking medical treatment - would not be impacted by the new rules.

Of the new nationalities facing visa restrictions, Nigerians account for the most immigration to the US.

According to US government statistics, the State Department issued 8,018 immigrant visas to Nigerians in the fiscal year 2018.

Kyrgyzstan and Sudan have large Muslim majorities, while around 50% of people in Nigeria and Eritrea are Muslim. Tanzania also has a sizable Muslim community.

What is the travel ban?

Mr Trump signed a controversial travel ban just seven days after taking office in January 2017, arguing it was vital to protect Americans.

The ban initially excluded people from seven majority-Muslim countries but the list was modified following a series of court challenges.

It now restricts citizens of Iran, Libya, Somalia, Syria, Yemen, Venezuela and North Korea.

While the government has suspended most immigrant and non-immigrant visas to applicants from those countries, exceptions are available for students and those with "significant contacts" in the US.