The Group B rivals meet in southeastern Nigerian city Uyo this Friday and in Cameroonian capital Yaounde three days later.
Nigeria top the African “group of death” with six points after two rounds, African champions Cameroon have two and Zambia and Algeria one.
“If we still want to have a chance of qualifying, we must take at least four points off Nigeria,” the Belgian told reporters.
“Should we fail to achieve that target, I do not think it will be possible to qualify for Russia.”
Broos is hoping for an away draw and a home victory — results that would reduce the gap between the countries to one point.
The remaining two qualifying rounds are scheduled for October and November with only the five group winners in Africa securing places at the global football showcase.
“My players need little or no reminding of how important the matches against Cameroon are,” said Germany-born Nigeria coach Gernot Rohr.
While Nigeria were fast out of the blocks late last year to defeat Zambia and Algeria, Cameroon did well to draw in Algeria only to be held at home by Zambia.
Bottom-of-the-table Algeria dumped midfielder Sofiane Feghouli, a recent Galatasaray signing from West Ham United, for a must-win showdown with Zambia in Lusaka Saturday.
Algeria, Cameroon and Nigeria qualified for the 2010 and 2014 World Cup tournaments, but only one of the three countries can make it to Russia.
Ghana, seeking a fourth consecutive World Cup appearance, host Congo Brazzaville in Kumasi Friday needing maximum points to have any realistic hope of overtaking Group E leaders Egypt.
The “Pharaohs” have six points and the “Black Stars” only one with upstarts Uganda between them on four.
Ghana coach Kwesi Appiah called up long-time campaigners like the Ayew brothers, Andre and Jordan, and Asamoah Gyan, and also named five uncapped players.
Egypt are away to Uganda Thursday in a repeat of a 2017 Africa Cup of Nations group match in Gabon won 1-0 by the north Africans thanks to a last-gasp Abdallah Said goal.
Said is in the squad, and so is 44-year-old goalkeeper Essam El Hadary, as Egypt seek a win after losing the 2017 Nations Cup final to Cameroon and a 2019 qualifier in Tunisia.
South Africa accept that hopes of a fourth World Cup appearance could hinge on defeating Cape Verde twice in Group D, starting in Praia Friday.
Although an often windy national stadium can help the Cape Verdean “Blue Sharks”, they have lost competitive matches there against Morocco, Libya and Uganda since last year.
Burkina Faso, who lead South Africa on goal difference, are away Saturday in Dakar to Senegal, whose formidable attack boasts in-form Liverpool winger Sadio Mane.
Gabon will lack 2015 African Footballer of the Year Pierre-Emerick Aubameyang when they host Group C pacesetters the Ivory Coast in Libreville Saturday.
Asked why the Borussia Dortmund striker was missing, a Gabonese football official said: “The coach (Spaniard Jose Antonio Camacho) has chosen players who were available.”
Ivory Coast have four points, Gabon and Morocco two and Mali one with the Herve Renard-coached Moroccans hosting the Malians in Rabat Friday.
Group A appears to be a straight fight between Tunisia and the Democratic Republic of Congo after they defeated Guinea and Libya in previous rounds.
The first top-of-the-table meeting is set for Friday in Rades and the Congolese will be handicapped by the absence of injured Everton flier Yannick Bolasie.