Learn French: Ils ne refusent pas l'accord à Paris ? Si. - They aren't refusing the agreement in Paris? Yes, they are.

When someone asks a negative question (using ne...pas) and you want to contradict them by saying yes, you must use si. If you used oui, a French speaker might be confused about whether you agree with the negative statement or are trying to contradict it.

Does si mean anything else in French?

Yes! You have probably already seen si used to mean if, such as in s'il vous plaît. Using it to mean yes is a special rule specifically for answering negative questions or contradicting negative statements.

How would I reply if I wanted to agree and say no, they aren't refusing it?

You would simply answer with non. Just like in English, if you want to agree with the negative question (meaning they are indeed not refusing it), you use non.

Can I use est-ce que to ask a negative question like this?

Yes, you could say Est-ce qu'ils ne refusent pas l'accord à Paris ? However, simply using the normal statement word order with a question mark at the end (and a rising intonation when speaking) is very common and natural in everyday French.