From Washington, this is VOA news. I'm Michael Brown reporting.

The White House said Monday, "Don't expect President Donald Trump to issue any red line warnings to North Korea." Sean Spicer is a White House spokesman: "I think when we talked about, you know, the use of red lines in the past with respect to Syria -- the president's red line, you know -- that drawing red lines hasn't really worked in the past. He holds his cards close to the vest, and I think you're not going to see him telegraphing how he's going to respond to any military or other situation going forward. That's just something that he believes has not served us well in the past. We did this with Mosul, where we started to talk about what the action will be months in advance. And it really gives the intended recipient of action a heads-up as to what's going on."

Spicer's comment came after U.S. Vice President Mike Pence Monday issued a strong warning to North Korea that President Trump will use military force if needed to deal with Kim Jong Un's government's escalating nuclear threat.

"For more than two decades, the United States and our allies have worked to peacefully dismantle North Korea's nuclear program, and alleviate the suffering of their people. But at every step of the way, North Korea answered our overtures with willful deception, broken promises and nuclear and missile tests. The era of strategic patience is over."

The vice president is visiting South Korea as the start of a four-nation Asia tour to reinforce the U.S. commitment to allies and this increasingly volatile region and build international support for increasing pressure on the North to end its nuclear and ballistic missile development programs.

Many in South Korea have grown concerned that the U.S. might take unilateral military action against the North that could plunge the region into war.

From here in Washington, this is VOA news.

Nigeria's defense minister warns it may take years to find all the Chibok girls kidnapped by the militant group Boko Haram.

General Manir Dan Ali said the military is searching Boko Haram's hideouts in the Sambisa Forest, which covers three states in northeastern Nigeria.

Ali spoke to VOA as activists mark the third anniversary of the girls' abductions. Of the 276 girls taken three years ago from their school, 195 girls are still missing.

International monitors are questioning the fairness of Sunday's referendum in Turkey, saying it was contested on an uneven playing field. The referendum, which created a powerful executive presidency from the current parliamentary system, only narrowly passed and opponents are seeking a revote. Dorian Jones reports from Istanbul.

At a news conference in Ankara, monitors from the Organization for Security and Cooperation in Europe said the "No" campaign faced numerous obstacles, including a lack of freedom of expression, intimidation and access to the media. The OSCE also alleged misuse of administrative resources by President Recep Tayyip Erdoğan.

Cezar Florin Preda of the monitoring group highlighted the controversial decision to allow the use of ballots which did not have an official stamp marking them as legitimate.

"The Supreme Election Board issued instructions late in the day that significantly changed the validity criteria."

The U.S. State Department called on Turkey to protect basic rights and freedoms as authorities work to resolve the contested results.

Dorian Jones, of VOA news, Istanbul.

A spokesman of the U.S.-led coalition to defeat the Islamic State group said on Monday that there were fewer than 1,000 Islamic State fighters left in Mosul.

Speaking in Baghdad, Colonel John Dorian did not give a timeline for the operation to retake Mosul.

"Yea, this is a very dangerous and difficult part of the battle... The fact that the enemy is using civilians as human shields has made things very difficult. But the Iraqi security forces continue to advance. It's a little bit each day and eventually the enemy is gonna be crushed in Mosul."

Dorian said that coalition aircraft have dropped more than 20,000 bombs since the beginning of the operation, ???destroyed more than 220 suicide car bombs, which he defined as the Islamic State group's most dangerous tactic.

Iraqi security forces retook eastern Mosul from the Islamic group at the end of January, continue fighting for control of the western half of the city.

For details on much more news, we invite you to join us at our website. That is voanews.com.

I'm Michael Brown reporting here in Washington.

That's the latest world news from VOA.