From Washington, this is VOA news. I'm David DeForest reporting. There are reports that hundreds of rebels are preparing an offensive to seize control of the Syrian town of Jarablus now under control of the Islamic State group.

Turkey, which has been shelling the border town, vowed Tuesday to give full support to efforts to free it from IS forces.

A rebel incursion into Syria from Turkey would likely result in a conflict with U.S.-backed Kurdish forces who also want to force IS from the area but are opposed by Turkey.

The State Department says Turkey has requested the extradition of U.S.-based cleric Fethullah Gulen, who was accused of involvement in a coup attempt earlier this year.

Spokesman Mark Toner said the request was not related to the attempted coup but to other issues for which Gulen was being sought by Turkish authorities.

The request comes just ahead of a visit to Ankara Wednesday by U.S. Vice President Joe Biden. During the visit, Biden is expected to discuss the extradition request and other issues.

The U.S. military says an American soldier died Tuesday as a result of wounds sustained during operations in Afghanistan's Helmand province. Another was wounded and is in stable condition. Six Afghan soldiers were wounded in the operation near Lashkar Gah.

The U.S. military has recently deployed about 100 troops to Lashkar Gah to assist Afghan forces in their bid to defend the besieged city.

U.N. Secretary-General Ban Ki-moon warned Tuesday that there are no global treaties to prevent biological weapons attacks.

Speaking to the Security Council in New York, Ban said the world is not adequately prepared to deal with such an attack. He urged the Council to look at ways to strengthen Resolution 1540 which set up barriers to keep weapons of mass destruction out of the hands of non-state actors.

This is VOA news.

United Nations chief Ban Ki-moon has appointed former Dutch general Patrick Cammaerton to investigate actions by U.N. peacekeepers in South Sudan's capital last month.

A spokesman says the inquiry will look into reports of attacks on civilians and sexual violence in Juba and whether U.N. forces responded properly to the incidents.

Nigeria's air force Tuesday claimed to have killed Boko Haram leader Abubakar Shekau and several other high-ranking members of the group.

A spokesman for the Nigerian air force says Shekau was fatally wounded in the shoulder when government planes bombed the Boko Haram jungle stronghold in the Sambisa forest Friday.

Boko Haram has not immediately [claimed] responded to the claims.

The World Health Organization and partners are scaling up emergency operations in northeast Nigeria to reach people affected by Boko Haram. Lisa Schlein takes a look.

The World Health Organization recently declared northeastern Nigeria an organizational Grade 3 emergency. This is the highest emergency level ascribed to a humanitarian crisis, putting this region on a par with the dire situations in Syria, Yemen, Iraq and South Sudan.

WHO describes the health conditions in needs of 800,000 people who have only recently become accessible to the outside world as nothing short of catastrophic.

Lisa Schlein, Geneva.

On what is likely his last trip to Nigeria as secretary of state, John Kerry told an audience at the palace of the nation's top Muslim leader that extremist groups will only be stopped when governments cut down on corruption and offer opportunities to their young people.

Kerry linked the fight against groups, such as Boko Haram, to Nigeria's long-running struggle to curb corruption that analysts say keeps most of its 170 million people living in poverty.

U.S. President Barack Obama today visited Louisiana, where floods have ravaged much of the southern part of the state.

The president hoped to reassure flood victims that his administration has made recovery a top priority.

"I come here, first and foremost, to say that the prayers of the entire nation are with everybody who lost loved ones. We are heartbroken by the loss of life."

Storms and floods earlier this month have claimed the lives of at least 13 people.

Republican presidential candidate Donald Trump has abandoned a call to deport all 11 million illegal immigrants living in the United States. Instead, Trump says he will send convicted criminals back to their home countries and handle other immigration residency disputes much the way the country does now but with greater urgency.

Trump says he is not "flip-flopping" on the deportation issue and has not ended a vow to build a wall along the country's southern border.

From the VOA news center in Washington, I'm David DeForest.

That's the latest world news from VOA.